Code Quality: Simplify user permission check for importing new users.

This removes a redundant conditional in the `check_import_new_users()` function, simplifying its logic to directly return the result of the permission check.

Follow-up to [https://mu.trac.wordpress.org/changeset/1829 mu:1829], [39945].

Props Soean.
See #64238.

git-svn-id: https://develop.svn.wordpress.org/trunk@62086 602fd350-edb4-49c9-b593-d223f7449a82