Remove some TODOs from gdb.cp tests

Remove some TODOs from gdb.cp tests

This patch removes many TODOs from the gdb.cp tests.
Going through the patch:

* bs15503.exp - these have been commented out forever and rely on
  libstdc++ debuginfo.  It's better to just remove these.

* classes.exp - the test is wrong, I think, according to the C++ ABI
  that gdb understands; and the test can be fixed and comments removed
  with a simple change to the code.

* ctti.exp - there's no need to bail out any more, as the test works.

* exception.exp - the code relying on the line numbers can't work,
  because gdb never prints that message anyway.

Reviewed-By: Bruno Larsen <blarsen@redhat.com>