Remove unnecesary "header" block redeclaration

Remove unnecesary header block redeclaration on Magento Luma Theme, which made impossible to set the header template through custom modules.

Example: 

- Create new module.
- Create new default.xml and try to use referenceBlock to set the template.
- Doesn't work, because Luma theme XML files are compiled after all modules and overrides the custom module modifications.

Removed because this block was already created in module Magento_Theme, so this redeclaration is just unnecesary

Author: samuel27m

app/design/frontend/Magento/luma/Magento_Customer/layout/default.xml

@@ -15,11 +15,6 @@
                 </arguments>
             </block>
         </referenceBlock>
-        <block class="Magento\Theme\Block\Html\Header" name="header" as="header">
-            <arguments>
-                <argument name="show_part" xsi:type="string">welcome</argument>
-            </arguments>
-        </block>
         <move element="header" destination="header.links" before="-"/>
         <move element="register-link" destination="header.links"/>
         <move element="top.links" destination="customer"/>