In api-paste.ini, legacy_ratelimit has been unused since the patch
I1476b2e364032d7c98f71df0cd61f1d1c19e005d. And there is a dependency
of limits module on legacy v2 API code because the legacy_ratelimit
remains.
This patch removes these code for removing the dependency.
Partially implements blueprint remove-legacy-v2-api-code
Change-Id: I4ecf1c9bd8b419769d474ddbb8116fe834199843