Investigate whether stages pipeline variable can be removed

[mthalman]

AzDO now supports the selection of which stages should be run when queuing a build. In theory, this would mean the stages variable could be obsolete. This should be investigated to see if we can now get rid of this variable.

[MichaelSimons]

Current the buildtools-prereqs repo does not have any tests. We could add an no-op run-tests to accommodate this.

The benefit of this is that it would help simplify the build condition logic.