[gdb/python] Drop ATTRIBUTE_UNUSED on py_initialize_catch_abort

I added ATTRIBUTE_UNUSED to py_initialize_catch_abort as a quick fix to deal
with it being unused for PY_VERSION_HEX >= 0x030a0000, but forgot to fix this
before committing.

Fix this now, by removing the attribute and using
'#if PY_VERSION_HEX < 0x030a0000' instead.

Tested on aarch64-linux.

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