[linker] Use mono's linker submodule

Instead of having own submodule. This way we unify with xamarin-macios
as it also uses mono's linker submodule.

Author: radekdoulik

.gitmodules

@@ -29,10 +29,6 @@
 	path = external/llvm
 	url = https://github.com/mono/llvm.git
 	branch = master
-[submodule "external/linker"]
-	path = external/linker
-	url = https://github.com/mono/linker.git
-	branch = master
 [submodule "external/proguard"]
 	path = external/proguard
 	url = https://github.com/xamarin/proguard.git

Configuration.props

@@ -56,7 +56,7 @@
     <MonoSourceDirectory>$(MSBuildThisFileDirectory)external\mono</MonoSourceDirectory>
     <MonoRequiredMinimumVersion Condition=" '$(MonoRequiredMinimumVersion)' == '' ">5.4.0</MonoRequiredMinimumVersion>
     <MonoRequiredDarwinMinimumVersion>$(MonoRequiredMinimumVersion).147</MonoRequiredDarwinMinimumVersion>
-    <LinkerSourceDirectory>$(MSBuildThisFileDirectory)external\linker</LinkerSourceDirectory>
+    <LinkerSourceDirectory>$(MSBuildThisFileDirectory)external\mono\external\linker</LinkerSourceDirectory>
     <OpenTKSourceDirectory>$(MSBuildThisFileDirectory)external\opentk</OpenTKSourceDirectory>
     <LibZipSourceDirectory Condition=" '$(LibZipSourceDirectory)' == '' ">$(MSBuildThisFileDirectory)external\libzip</LibZipSourceDirectory>
     <LibZipSharpSourceDirectory Condition=" '$(LibZipSharpSourceDirectory)' == '' ">$(MSBuildThisFileDirectory)external\LibZipSharp</LibZipSharpSourceDirectory>