Make all entities (such as monsters and turrets) use physics logic handled by Xonotic rather than the engine, allowing for future advancements in movement and gameplay

Migrating to QuakeC-driven physics by removing the cl_useenginerefdef and sv_qcphysics cvars previously used for testing. The QuakeC implementations are now the default and only options.

cl_useenginerefdef has been off for long enough (soon to be part of a release) that it can be safely assumed the QuakeC implementation is fine.

The change to remove sv_qcphysics is a little less certain; while QuakeC physics have undergone extensive testing with player entities, this change makes it so ALL entities are QC-driven.
No behavioural changes are intended, but much testing will be required, especially in maps with lots of moving platforms.