Remove CONTRIBUTING.md policy requiring tests be run for vendored dependencies (#6586)

This policy existed to ensure that, when updating a dependency to a
non-release-tagged commit, we weren't accidentally bringing in a
broken version of that dependency. Since it was instituted, we have
greatly reduced the number of non-versioned dependencies. And we
have extensive tests of our own that should detect if any of the behavior
we use from one of these dependencies is broken. Therefore this
requirement has fallen by the wayside.

Update the document to reflect current practices.

Author: pgporada

docs/CONTRIBUTING.md

@@ -20,10 +20,6 @@ guidelines for Boulder contributions.
 * You cannot review your own code.
 * If a branch contains commits from multiple authors, it needs a reviewer who
   is not an author of commits on that branch.
-* If a branch contains updates to files in the vendor/ directory, the author is
-  responsible for running tests in all updated dependencies, and commenting in
-  the review thread that they have done so. Reviewers must not approve reviews
-  that have changes in vendor/ but lack a comment about tests.
 * Review changes to or addition of tests just as rigorously as you review code
   changes. Consider: Do tests actually test what they mean to test? Is this the
   best way to test the functionality in question? Do the tests cover all the