delete target dbug/picobug/dink32/m32r/mon2000 ?

All of these targets use gdb/monitor.c, which has bit rotted
years ago (I'd guess around ~6), and nobody seems to
have noticed.

I propose deleting them (along with monitor.c):

| target         | source               |
|----------------+----------------------|
| target dbug    | gdb/dbug-rom.c       |
| target picobug | gdb/microblaze-rom.c |
| target dink32  | gdb/dink32-rom.c     |
| target m32r    | gdb/m32r-rom.c       |
| target mon2000 | gdb/m32r-rom.c       |

Thanks,
Pedro Alves