Remove 'if' from GDB_PY_HANDLE_EXCEPTION

This removes the embedded 'if' from GDB_PY_HANDLE_EXCEPTION and
GDB_PY_SET_HANDLE_EXCEPTION.  I believe this 'if' was necessary with
the old gdb try/catch macros, but it no longer is: these should only
ever be called from a 'catch' block, where it's already known that an
exception was thrown.

Simon pointed out, though, that in a few spots, these were in facts
called outside of 'catch' blocks.  This patch cleans up these spots.
I also found one spot where a redundant 'return nullptr' could be
removed.

gdb/python/py-infthread.c

@@ -214,7 +214,6 @@ thpy_get_ptid_string (PyObject *self, void *closure)
   catch (const gdb_exception &except)
     {
       GDB_PY_HANDLE_EXCEPTION (except);
-      return nullptr;
     }
 }