Make Special:MIMESearch non-expensive, again

Make Special:MIMESearch non-expensive, again

The queries here are using a good-enough index and are not expensive
(since 3342fafb), taking a second or two at most on Wikimedia Commons.
However, they use inefficient offset+limit paging, which caused the
issues described in T107265 and resulted in the page being marked as
expensive again in 77014c76. But since we limited offset to 10000 in
e33d6f45, this should be fine to enable again.

Bug: T107265
Change-Id: I65ec541dd32f1e23488f5d5e1886332af2a55744