Retire obsolete GDB 6.1.1

Retire obsolete GDB 6.1.1

GDB 6.1.1 was released in June 2004 and is long obsolete. It does not
support all of the architectures that FreeBSD does, and imposes
limitations on the FreeBSD kernel build, such as the continued use of
DWARF2 debugging information.

It was kept (in /usr/libexec/) only for use by crashinfo(8), which
extracts some basic information from a kernel core dump after a crash.
Crashinfo already prefers gdb from port/package if installed.

Future work may add kernel debug support to LLDB or find another path
for crashinfo's needs, but in any case we do not want to ship the
excessively outdated GDB in FreeBSD 13.

Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27610