be set always.

trx_rollback_active(): Split from trx_rollback_or_clean_all_without_sess().

row_undo_dictionary(): Do not return a value.  Assert that all operations
succeed.

row_merge_drop_index(): Remove bogus comment about void return value.

trx_dummy_sess: Move the declaration from trx0roll.h to trx0trx.h,
because the variable is defined in trx0trx.c.