Remove P_FindSectorFromLineTag and P_FindLineFromLineTag
In UDMF, the tag of a linedef/sector/thing is never used for calling something, only for being called. This makes P_FindSectorFromLineTag and P_FindLineFromLineTag useless, so I replaced all occurrences with P_FindSectorFromTag and P_FindLineFromTag. Based on !900 (merged), so that one should be merged first.