gdb: remove unnecessary notification of executable_changed observer

gdb: remove unnecessary notification of executable_changed observer

This commit continues the work of the previous two commits.

My goal, in the next couple of commits, is to expose the
executable_changed observable in the Python API as an event.  However,
before I do that I want to remove the use of the executable_changed
observable from the reread_symbols function in symfile.c as this use
isn't directly associated with a change of the executable file, and so
seems wrong.

In the previous two commits I have removed all users of the
executable_changed observer as I believe those users can, and should,
actually be listening for the new_objfile observable instead, so now
there are no users of the executable_changed observable.

As such, I think removing the use of executable_changed from the
function reread_symbols is perfectly safe, and correct.  At this point
the executable has not been changed, so we shouldn't be sending an
executable_changed notification, and, as there is nobody listening to
this observable, we can't break anything by removing this call.

There should be no user visible changes after this commit.

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