Build/platform matrix remove redhat.6 from dependencies (#28213)

* remove redhat.6.amd64 from platform matrix yml.

* minor: platform-matrix.yml - removed extra space

* remove dependency for removed platform

Author: vseanreesermsft

eng/pipelines/internal.yml

@@ -57,7 +57,6 @@ stages:
         - build_Linux_arm64_release
         - build_Linux_musl_x64_release
         - build_Linux_musl_arm64_release
-        - build_Linux_rhel6_x64_release
         - build_Linux_x64_release
         - build_OSX_x64_release
         - build_Windows_NT_x64_release