msgpack: remove dead code handling invalid MP_EXT

We have a few functions that decode MsgPack data assuming it was
previously checked with mp_check(). This means it's safe to expect
that MP_EXT contains valid data because we install a custom checker
for MP_EXT in msgpack_init. So let's replace errors with assertions,
removing the dead code.

NO_DOC=code cleanup
NO_TEST=code cleanup
NO_CHANGELOG=code cleanup