t/lib/warnings/mg: Remove comment

The warning this refers to was removed (or moved to perl5db.pl) in
commit bbdae7b29367.
Fold t/lib/warnings/irs into mg

It took me a while to find these tests.  Most of these tests are arranged
by which C file they occur in.  (These warning tests were originally
added to make sure all warnings in all C files get tested.)
Make $/=-1 warning default like other dep warnings