rtr_pcur_getnext_from_path(): Remove a bogus assertion
that may cause a data races with buf_LRU_block_free_non_file_page().

If my_latch_mode == BTR_MODIFY_LEAF, we would have released all page
latches and buffer-fixes by invoking mtr->rollback_to_savepoint(1).
After this point, the btr_cur->page_cur.block is no longer valid and
must not be accessed.

Before 03ca6495 this assertion had
been disabled, because the preprocessor symbol UNIV_RTR_DEBUG
had never been enabled (except when explicitly specified in
CMAKE_CXX_FLAGS).

Reviewed by: Debarun Banerjee