gdb: remove deprecated_exec_file_display_hook and associated code

This commit removes deprecated_exec_file_display_hook and the
associated specify_exec_file_hook.

The specify_exec_file_hook is used to add a new hook function to
deprecated_exec_file_display_hook, but is only used from the insight
debugger.

I posted a patch to remove the use of specify_exec_file_hook from
insight, and instead use gdb::observers::executable_changed, this
patch can be found here (it has now been merged):

  https://inbox.sourceware.org/insight/6abeb45e97d9004ec331e94cf2089af00553de76.1702379379.git.aburgess@redhat.com/T/#u

With this merged we can now cleanup the GDB side as this code is now
unused.

There should be no user visible changes after this commit.

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