[gdb/python] Eliminate GDB_PY_SET_HANDLE_EXCEPTION

[gdb/python] Eliminate GDB_PY_SET_HANDLE_EXCEPTION

Result of:
...
$ search="GDB_PY_SET_HANDLE_EXCEPTION ("
$ replace="return gdbpy_handle_gdb_exception (-1, "
$ sed -i \
    "s/$search/$replace/" \
    gdb/python/*.c
...

Also remove the now unused GDB_PY_SET_HANDLE_EXCEPTION.

No functional changes.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>