fts_create_doc_id(): Remove.

row_mysql_convert_row_to_innobase(): Implement the logic of
fts_create_doc_id(). Reuse a buffer for the hidden FTS_DOC_ID.

row_get_prebuilt_insert_row(): Allocate a buffer for the hidden
FTS_DOC_ID at the end of prebuilt->ins_upd_rec_buff.