Remove ARMv6 from all platformgroups, matching s390x (#64535)

Turns out CoreCLR uses platformgroups a lot for managing
extra pipelines, and ignores valid subsets for these pipelines.
We should just... not try and get involved in platformgroups.

Author: directhex

eng/pipelines/common/platform-matrix.yml

@@ -53,7 +53,7 @@ jobs:
         ${{ insert }}: ${{ parameters.jobParameters }}
 
 # Linux armv6
-- ${{ if or(containsValue(parameters.platforms, 'Linux_armv6'), or(and(ne(parameters.runtimeFlavor, 'mono'), in(parameters.platformGroup, 'gcstress')), and(eq(parameters.runtimeFlavor, 'mono'), in(parameters.platformGroup, 'all', 'gcstress')))) }}:
+- ${{ if containsValue(parameters.platforms, 'Linux_armv6') }}:
   - template: xplat-setup.yml
     parameters:
       jobTemplate: ${{ parameters.jobTemplate }}