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
diff --git a/core/main.mk b/core/main.mk
index b7b6cd0..ecdbcbd 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -194,6 +194,8 @@
 $(error stopping)
 endif
 
+# These are the valid values of TARGET_BUILD_VARIANT.
+INTERNAL_VALID_VARIANTS := user userdebug eng
 ifneq ($(filter-out $(INTERNAL_VALID_VARIANTS),$(TARGET_BUILD_VARIANT)),)
 $(info ***************************************************************)
 $(info ***************************************************************)