P_CeilingzAtPos changes

Changes one thing about the P_CeilingzAtPos function merge request I submitted a couple of months ago. All it does is removes quicksand FOF functionality. Originally I just copypasted the quicksand checks from P_FloorzAtPos, but then later on realised it never had any purpose, since all the check in P_FloorzAtPos is there for is to return how deep into quicksand you are.