remove "base_workspace" entry from .gitignore list

Since commit e87849b: ("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.

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

.gitignore

@@ -6,7 +6,6 @@
 /.project
 /.settings
 /WORKSPACE.user.bzl
-/base_workspace/*
 /bazel-bazel
 /bazel-bin
 /bazel-genfiles