[TASK] Move MysqlFulltextIndexHook to EXT:indexed_search_mysql

The hook MysqlFulltextIndexHook should be moved from indexed_search to
indexed_search_mysql because it is only used if indexed_search_mysql is activated.

Resolves: #62201
Releases: master
Change-Id: I11d61f771abb2d2431c26156fb1d6e0104cba5c9
Reviewed-on: http://review.typo3.org/33296
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>