[DocsUI M3] Remove search bar

Remove the search bar (searchbar_title) which only appeared
in Recents so that the backup search icon (option_menu_search) shows
instead.

Bug: 409934594
Test: (with flag on and off) atest FilesActivityUiTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ibc01d85a8223fad6af12761bdc0c817ff29c3009