Get rid of TrxUndoRsegs code

The code is not necessary after MDEV-12289 implementation.

TrxUndoRsegs is wrapper for vector of trx_rseg_t*. It has two
constructors, both initialize the vector with only one element. And they
are used to push transactions rseg(the singular) to purge queue. There is
no function to add elements to the vector. The default constructor is used
only for declaration of NullElement.

The TrxUndoRsegs was introduced in WL#6915 in MySQL 5.7 and. MySQL 5.7
would unnecessarily let the purge of history parse the
temporary undo records, and then look up the table (via a global hash
table), and only at the point of processing the parsed undo log record
determine that the table is a temporary table and the undo record must be
thrown away.

In MariaDB 10.2 we have two disjoint sets of rollback segments (128 for
persistent, 128 for temporary), and purge does not even see the temporary
tables. The only reason why temporary tables are visible to other threads
is a SQL layer bug (MDEV-17805).