The check causes external plugin's quotas fields unknown
exception because the quotas fields are not part of
QUOTA_FIELDS which only includes the fields for
cinder, neutron and nova.
Meanwhile the exception message throws TypeError.
This commit removes the uncessary check. It also helps
fix the manila-ui's bug #1842119
Change-Id: If093f4ecf1e344792b347c5c338b441b1ab42a04
Closes-bug: #1798048
Closes-bug: #1842119