remove uncessary instructions f. contribution guide (#1454)

developers/contributing.md

@@ -89,11 +89,6 @@ sure to post a comment after pushing. The new commits will show up in the pull
 request automatically, but the reviewers will not be notified unless you
 comment.
 
-Before the pull request is merged, make sure that you squash your commits into
-logical units of work using `git rebase -i` and `git push -f`. After every
-commit the test suite should be passing. Include documentation changes in the
-same commit so that a revert would remove all traces of the feature or fix.
-
 Commits that fix or close an issue should include a reference like `Closes #XXX`
 or `Fixes #XXX`, which will automatically close the issue when merged.