Fix signature of `PyUpb_MessageMeta_Clear` (#17959)

A `tp_clear` function should have signature `int f(PyObject*)`. The presence of erroneous extra parameters leads to undefined behavior as indicated in the C specification 6.3.2.3.8. In WebAssembly builds, this causes crashes. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf#page=60

Closes #17959

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/17959 from hoodmane:fix-message-meta-clear-sig b618f806a9c9a7fec79caedb218c32991c2fbf98
PiperOrigin-RevId: 669639153