gdb/testsuite: Prepare for DejaGnu 1.6.2

gdb/testsuite: Prepare for DejaGnu 1.6.2

Changes in DejaGnu 1.6.2 mean that our testsuite will no longer run.
This is because of some confusion over how the gdb.exp file is
handled.

The gdb.exp file is really the tool init file, which is loaded from
within the DejaGnu core, and it should not be loaded directly from any
other file in the testsuite.

DejaGnu tries to prevent the same library being loaded twice by
remembering the names of library files as they are loaded.  Until
recently loading the tool init file in DejaGnu was very similar to
loading a library file, as a result, loading the gdb.exp tool init
file simply recorded 'gdb.exp' as having been loaded, future attempts
to load 'gdb.exp' as a library would then be ignored (as the file was
marked as already loaded).

DejaGnu has now changed so that it supports having both a tool init
file and a library with the same name, something that was not possible
before.  What this means however is that when the core loads the
'gdb.exp' tool init file it no longer marks the library 'gdb.exp' as
having been loaded.  When we then execute 'load_lib gdb.exp' we then
try to reload the 'gdb.exp' file.

Unfortunately our gdb.exp file can only be loaded once.  It use of
'rename cd builtin_cd' means that a second attempt to load this file
will fail.

This was discussed on the DejaGnu list here:
   http://lists.gnu.org/archive/html/dejagnu/2019-03/msg00000.html

and the suggested advice is that, unless we have some real requirement
to load the tool init file twice, we should remove calls to 'load_lib
gdb.exp' and rely on DejaGnu to load the file for us, which is what
this patch does.

I've tested with native X86-64/GNU Linux and see no regressions.

gdb/testsuite/ChangeLog:

	* config/default.exp: Remove 'load_lib gdb.exp'.
	* config/monitor.exp: Likewise.
	* config/sid.exp: Likewise.
	* config/sim.exp: Likewise.
	* config/slite.exp: Likewise.
	* config/unix.exp: Likewise.
	* gdb.base/default.exp: Remove unhelpful comment.