extend/os/mac/keg_relocate: remove HOMEBREW_RELOCATE_METAVARS

[carlocab]

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It's not being used, and it breaks formulae when it is.

Plus do some extra refactoring.

[BrewTestBot]

Review period will end on 2021-05-13 at 03:39:58 UTC.

[BrewTestBot]

Review period ended.