Delete --no-main from COMP_FLAGS.

main.c was deleted in commit 268b6fff783f709e40e4b71b3c2a5cfccafceac9
and --no-main was removed from MLFLAGS, but another instance occurs in
COMP_FLAGS.  The latter only affects the mmc --make build process,
not mmake.

compiler/COMP_FLAGS.in:
	Delete --no-main from COMP_FLAGS to restore compilation
	with mmc --make.