gdb: remove arm simulator support

With commit:

  commit f8297737722d63ba4a23c8df21765ab5a8368d64
  Date:   Tue Mar 25 13:30:32 2025 +0000

      Delete the ARM sub-directory of the SIM directory.

The arm simulator was removed.  There's no point in keeping arm
simulator related code in GDB.  Lets remove it.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>