[DocsUI, Search]: Remove old use-search-v2 flag.

We switched to the read-write flag in ag/31790039. The old flag is no
longer needed.

Bug: 383412640
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_rw
Change-Id: Ib2699c28fe5a34063ffbd30dd9a0d18af72b2678