Simplify gnu/usr.bin Makefile conditions

After r307655 MK_GDB is forced to no if MK_BINUTILS is no, and similarly
MK_GROFF is forced to no if MK_CXX is no, so we can remove nested
conditionals.

Reviewed by: bapt, brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8287