Remove the PRODUCT-* goal

I've switched all the build server configs to using TARGET_PRODUCT /
TARGET_BUILD_VARIANT and explicit goals instead.

Remove tools/check_builds.sh which relied on this, but hasn't been
touched in a long time.

Test: m PRODUCT-test
Test: treehugger
Change-Id: If5f8c714567b33aeb38223c176ca24ea649eb57d