gdb: remove QNX Neutrino support

Remove the support for the QNX Neutrino OS (tdep and native bits).  This
has been unmaintained for years, and we don't have a way to see if it
works (or even builds, for the native parts).  Without somebody actively
maintaining it, this is just a burden for developers, especially that
this port does a few weird unique things that require reasoning about
when doing big change.

Support for GDBserver was removed in 2020, commit 613f149a
("gdbserver: remove support for Neutrino").

Change-Id: I4e25ec26ab06636629adebd02ceb161ee31c232d
Approved-by: Kevin Buettner <kevinb@redhat.com>