gdb: remove one user of the executable changed observer

My goal for the next few commits is to expose the executable_changed
observable from the Python API.

However, there is call to the executable_changed observable in the
reread_symbols function (in symfile.c), and this doesn't actually
correspond to the executable changing.  My idea then, is to remove
this use of the executable_changed observable, but, before I can do
that, I need to check that nothing is going to break, and that
requires my to think about the current users of this observable.

One current user of executable_changed is in symtab.c.  We add an
executable_changed observer that calls:

  set_main_name (nullptr, language_unknown);

to discard all information about the main function when the executable
changes.

However, changing the executable doesn't actually change the debug
information.  The debug information changes when the symbol-file
changes, so I think this observer is in slightly the wrong place.

The new_objfile observable is (unfortunately) overloaded, it is called
when a new objfile is loaded, and also (when its argument is nullptr),
when all debug information should be discarded.

It turns out that there is already a new_objfile observer in
symtab.c.  I propose that, when the argument is nullptr (indicating
all debug info should be discarded), that we should call set_main_name
to discard the information about the main function.  We can then
remove the executable_changed observer from symtab.c.

All tests still pass, and, in my local world, I added some debug
printf calls, and I think we are still discarded the main information
everywhere we need to.

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