Remove if(isGV_with_GP(PL_defoutgv)) checks from mg.c

Remove if(isGV_with_GP(PL_defoutgv)) checks from mg.c

Commit 099be4f1d added code to cope with this:

    my $x = *STDERR; select($x); $x = 1;

which would cause PL_defoutgv to hold something other than a GV,
resulting in various crashes.

Commit 2acc3314 changed the way rv2gv works on fake globs, and inad-
vertently fixed this problem, too, so PL_defoutgv can no longer end up
holding something other than a GV.

So the code that checks if(isGV_with_GP(PL_defoutgv)) can go.