Expunge `PYOP2_NO_FORK_AVAILABLE`

See previous PyOP2 issues: https://github.com/OP2/PyOP2/issues/684 and https://github.com/firedrakeproject/firedrake/issues/3820

Note that if forking becomes an issue again the "proper" solution is to probably use a non-forking JIT compiler like cppyy. See https://github.com/OP2/PyOP2/pull/614 for a (very old) WIP of this.