dict_index_t::db_trx_id(): Return the position of DB_TRX_ID.
Only valid for the clustered index.

dict_index_t::db_roll_ptr(): Return the position of DB_ROLL_PTR.
Only valid for the clustered index.

dict_index_get_sys_col_pos(): Remove. This was performing unnecessarily
complex computations, which only made sense for DB_ROW_ID, which would
exist either as the first field in the clustered index or as the last
field in a secondary index (only when a DB_ROW_ID column is materialised).

row_sel_store_row_id_to_prebuilt(): Remove, and replace with simpler code.

row_upd_index_entry_sys_field(): Remove.

btr_cur_log_sys(): Replaces row_upd_write_sys_vals_to_log().

btr_cur_write_sys(): Write DB_TRX_ID,DB_ROLL_PTR to a data tuple.