OSX: Remove RPATHs from git-annex binary, which are not needed, slow down startup, and break the OSX Sierra linker.

ghc 8.0.2 may make this unncessary, but it's not in a stackage version yet,
so put in a workaround.

Note that the linux builds already delete the RPATHs for similar reasons.

This commit was sponsored by Josh Taylor on Patreon.