Remove MachineIndependent configuration (#13180)

### Context

The `MachineIndependent` configuration enabled portable/xcopy MSBuild
toolsets for Mono environments by disabling machine-specific features
(registry, GAC, Visual Studio integration). With Mono EOL, this mode is
obsolete.

### Changes Made

- **Directory.Build.props**: Removed `MachineIndependent` from
`Configurations` property and deleted conditional PropertyGroup
- **src/Directory.BeforeCommon.targets**: Removed 12
`MachineIndependentBuild` conditions; feature flags now unconditionally
defined for .NET Framework targets:
- Registry access: `FEATURE_REGISTRY_TOOLSETS`, `FEATURE_REGISTRY_SDKS`,
`FEATURE_REGISTRYHIVE_DYNDATA`, `FEATURE_WIN32_REGISTRY`
  - GAC: `FEATURE_GAC`
  - Environment: `FEATURE_ENCODING_DEFAULT`, `FEATURE_INSTALLED_MSBUILD`
- Build features: `FEATURE_NODE_REUSE`, `FEATURE_NET35_TASKHOST`,
`FEATURE_SECURITY_PERMISSIONS`
- Simplified `FEATURE_VISUALSTUDIOSETUP` condition (removed
MachineIndependentBuild check)
- **MSBuild.sln**: Removed 260 lines of MachineIndependent platform
configurations

### Testing

- Full build passes (Debug/Release configurations)
- Framework and Build.Engine unit tests pass
- Bootstrap environment and sample builds functional

### Notes

All conditional features previously disabled in MachineIndependent mode
now enabled unconditionally. No behavioral change for existing
Debug/Release configurations.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Consider removing the MachineIndependent
configuration</issue_title>
>
<issue_description>https://github.com/search?q=repo%3Adotnet%2Fmsbuild%20MachineIndependentBuild&type=code
> 
> From what I know, this configuration is used to create a portable /
xcopy msbuild environment. Now that Mono is EOL and VS changing to a
rapid release cadence, I don't see much value in keeping this support
and the ifdefs that come with, alive.
> 
> @rainersigwald opinions?</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #13060

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Author: Copilot

Directory.Build.props

@@ -9,7 +9,7 @@
 
   <PropertyGroup>
     <Product>Microsoft® Build Tools®</Product>
-    <Configurations>Debug;Release;MachineIndependent</Configurations>
+    <Configurations>Debug;Release</Configurations>
   </PropertyGroup>
 
   <PropertyGroup>
@@ -69,15 +69,6 @@ ates https://learn.microsoft.com/en-gb/dotnet/fundamentals/syslib-diagnostics/sy
     <NoWarn Condition="'$(DotNetBuild)' == 'true'">$(NoWarn);NU5104;</NoWarn>
   </PropertyGroup>
 
-  <!-- Configuration MSBuild for portable (xcopy-install) toolsets: works on WinNT and linux/mac via Mono, isolates from machine environment:
-    uses only tools installed with it, ignores Registry and GAC and Visual Studio installations to provide the same build experience on all machines -->
-  <PropertyGroup Condition="'$(Configuration)' == 'MachineIndependent'">
-    <DebugSymbols Condition="'$(DebugSymbols)' == ''">true</DebugSymbols>
-    <Optimize Condition="'$(Optimize)' == ''">true</Optimize>
-    <DebugType Condition="'$(DebugType)' == ''">none</DebugType>
-    <MachineIndependentBuild>true</MachineIndependentBuild>
-  </PropertyGroup>
-
   <PropertyGroup>
     <DefaultItemExcludes>$(DefaultItemExcludes);*.log</DefaultItemExcludes>
     <DefaultItemExcludes>$(DefaultItemExcludes);*.binlog</DefaultItemExcludes>