Remove P_FindSectorFromLineTag and P_FindLineFromLineTag

MascaraSnakerequested to merge
find-sector-from-tag into next

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.

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