remove -a flag on stuart_build commands

[Javagedes]

-a is not a first class parameter for stuart_build, so mu_devops should not expect that it exists. If a platform has added a necessary -a argument, then they can pass it to the command in mu_devops through the build_flags for BuildPlatform.yml or extra_build_args for PrGate.yml

Integration

Any consumers of PlatformBuild.yml should remove the build_arch parameter. If it still needs to be passed, add the -a <arch> flag to the build_flags parameter.