gdb, gdbserver, python, testsuite: Remove MPX.

gdb, gdbserver, python, testsuite: Remove MPX.

GDB deprecated the commands "show/set mpx bound" in GDB 15.1, as Intel
listed Intel(R) Memory Protection Extensions (MPX) as removed in 2019.
MPX is also deprecated in gcc (since v9.1), the linux kernel (since v5.6)
and glibc (since v2.35).  Let's now remove MPX support in GDB completely.

This includes the removal of:
- MPX functionality including register support
- deprecated mpx commands
- i386 and amd64 implementation of the hooks report_signal_info and
  get_siginfo_type
- tests
- and pretty printer.

We keep MPX register numbers to not break compatibility with old gdbservers.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>