Remove support for the MPS garbage collector.

As Tomas By's recent emails suggest, this support is doing more harm than good,
by falsely implying to people that MPS is a viable alternative to the Boehm
collector. The MPS collector was only ever experimental, and never performed
as well as Boehm. MPS isn't even in the git repository on git hub. It was
stored in a separate CVS repository on mundula, and (as far as I know)
wasn't carried over to github. The code of MPS was last touched a long time
ago; I would be surprised if it worked on today's systems without changes.

Mmake.common.in:
Mmake.workspace:
RESERVED_MACRO_NAMES:
boehm_gc/Mmakefile:
compiler/add_pragma.m:
compiler/compile_target_code.m:
compiler/globals.m:
compiler/handle_options.m:
compiler/mercury_compile_mlds_back_end.m:
compiler/mlds_to_c.m:
compiler/options.m:
compiler/peephole.m:
doc/user_guide.texi:
library/benchmarking.m:
runtime/Mmakefile.m:
runtime/mercury.h:
runtime/mercury_conf_param.h:
runtime/mercury_grade.h:
runtime/mercury_heap.h:
runtime/mercury_init.h:
runtime/mercury_memory.h:
runtime/mercury_wrapper.[ch]:
scripts/canonical_grade.sh-subr:
scripts/init_grade_options.sh-subr:
scripts/mgnuc.in:
scripts/ml.in:
scripts/parse_grade_options.sh-subr:
util/mkinit.c:
    Remove all references to MPS.