Deprecation of dbx mode

[Tom Tromey]

Hi.  I proposed deprecating gdb's dbx mode for gdb 12, and then removing
it in gdb 13:

https://sourceware.org/pipermail/gdb-patches/2022-March/186306.html

Pedro suggested I mention that here.

My view is that dbx mode was always pretty superficial -- it just adds
some aliases and doesn't really try to emulate dbx -- and probably
largely unused.  It's never been well-documented.

Tom