libgcc/Makefile.in: Delete dead `MACHMODE_H` variable

libgcc/Makefile.in: Delete dead `MACHMODE_H` variable

This dates back to the creation of top-level `libgcc` in
fa9585134f6f58fa0d3da3ca4ad5493855aea2dc. I strongly suspect that this
does nothing.

Andrew Pinksi adds:

> So looking into this further, MACHMODE_H used part of LIBGCC_DEPS
> because of TM_H and r0-78222-gfa9585134f6f58 moved away from including
> tm.h from libgcc. It was copied over unused.

It is indeed used then.

(For background context, my overall goal here is hoping libgcc can depend on
fewer/no stuff that is generated by `gcc/Makefile`. This is me trying to
pluck some low-hanging fruit -- this is the only direct mention of
`insn-modes.h` in libgcc.)

libgcc/ChangeLog:

	* Makefile.in:  Delete dead `MACHMODE_H` variable