fix a bug that prevented git-annex init from working in a submodule

fix a bug that prevented git-annex init from working in a submodule

This is probably a reversion, but not sure what caused it. By the time
Annex.Init runs fixupUnusualReposAfterInit, another git-annex process has
at least sometimes already done the necessary fixups. (Eg, one run
indirectly by a git command.) But since the Repo is cached, it doesn't
realize and does them again. So, avoid crashing when git config --unset
fails.

This commit was sponsored by Jack Hill on Patreon.