Remove support for "target m32rsdi" and "target mips/pmon/ddb/rockhopper/lsi" ?

I need to change gdb/remote-mips.c and gdb/remote-m32r-sdi.c for
something related to the ongoing C++ conversion [1], and I wonder
if I can get rid of the files instead.

This would remove support for:

  | target            | source                |
  |-------------------+-----------------------|
  | target m32rsdi    | gdb/remote-m32r-sdi.c |
  | target mips       | gdb/remote-mips.c     |
  | target pmon       | gdb/remote-mips.c     |
  | target ddb        | gdb/remote-mips.c     |
  | target rockhopper | gdb/remote-mips.c     |
  | target lsi        | gdb/remote-mips.c     |

That is:

  - Remote M32R debugging over SDI.

  - Debugging boards using the MIPS remote debugging protocol
    over a serial line, and a few variants.

[1] - I'm looking at getting rid of immediate_quit, that is, stop
throwing from signal handlers, which these files rely on currently.

Thanks,
Pedro Alves