With core mesa doing runtime API checks, GLES overlay is no longer
needed. Make --enable-gles-overlay equivalent to --enable-gles[12].
There may still be places where compile-time checks are done. They
could be fixed case by case.
With the omit list gone, there are not too many differences in building
core mesa and ES overlay. Remove the mesa/es and build both of them in
src/mesa/Makefile.