Remove obsolete ffi entry from Gemfile.lock

It should have been removed as part of
https://gitlab.com/gitlab-org/gitlab/commit/78daae306669b956e9108555db3876f9656446df
since it is no longer a sub-dependency of `childprocess` 3.0.0.

qa/Gemfile.lock

@@ -37,7 +37,6 @@ GEM
       unf (>= 0.0.5, < 1.0.0)
     faker (1.9.3)
       i18n (>= 0.7)
-    ffi (1.9.25)
     gitlab-qa (4.0.0)
     http-cookie (1.0.3)
       domain_name (~> 0.5)