Delete residual support for the GCC backend.

scripts/Mmake.vars.in:
scripts/final_grade_option.sh-subr:
scripts/init_grade_options.sh-subr:
scripts/mmake.in:
scripts/parse_grade_options.sh-subr:
	As above.
Remove unused targets from Mmakefiles.

browser/Mmakefile:
compiler/Mmakefile:
library/Mmakefile:
mdbcomp/Mmakefile:
profiler/Mmakefile:
slice/Mmakefile:
ssdb/Mmakefile:
	Delete .ss and .pic_ss targets.
	These were only required for the GCC backend.