Since commit e87849b8: ("Remove base_workspace from bazel setup"),
this directory is not created anymore and thus we don't have to
have an entry for it in .gitignore.

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Change-Id: I71229e2b7762e2cc7f03e8852cbd06a4461ad168
Reviewed-on: https://bazel-review.googlesource.com/c/3340/
MOS_MIGRATED_REVID=119950175