openmw-34: fix pathgrid loading

Revert commit dd0cea21b0ead17a13198f3e584d343c6bb31875 "Implement overwriting pathgrid records"

This broke pathgrid loading in exterior cells due to an unexpected problem in the pathgrid record system (bug #2195). For the master branch a proper fix is proposed in pull request #394.

Upon the merging of this pull request the target version for bug #2175 "Pathgrid mods do not overwrite the existing pathgrid" should be set to openmw-0.35.