feature: rm useless code in sql_optimizer.cc

[hustjieke]

Is your feature request related to a problem? Please describe.

As we support delete & update now, the code next should be deleted.
Introduced on pr: ea99be9

    /*
      The primary key of the tianmu engine does not support delete and update statements.
      The following codes can be deleted after subsequent support
    */
    TABLE *const table= join_tab->table();
    bool tianmu_engine = table->s->db_type() ? table->s->db_type()->db_type == DB_TYPE_TIANMU: false;
    enum_sql_command sqlCommand = thd->lex->sql_command;
    bool tianmuDeleteOrUpdate = (tianmu_engine && (sqlCommand == SQLCOM_DELETE ||
                                          sqlCommand == SQLCOM_DELETE_MULTI ||
                                          sqlCommand == SQLCOM_UPDATE ||
                                          sqlCommand == SQLCOM_UPDATE_MULTI));

    if (!tianmuDeleteOrUpdate && update_ref_and_keys(thd, &keyuse_array, join_tab, tables, where_cond,
                            cond_equal, ~select_lex->outer_join, select_lex,
                            &sargables))

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[konghaiya]

These codes are intended to limit the use of primary keys, as the primary key functionality of the Tianmu engine is not comprehensive.
These codes can only be deleted after the functionality of the primary key is fully supported