The function was originally introduced by eb0804ef
MDEV-18553: MDEV-16327 pre-requisits part 1: isolation of LIMIT/OFFSET handling

set_unlimited had an overloaded notion of both clearing the offset value
and the limit value. The code is used for SQL_CALC_ROWS option to
disable the limit clause after the limit is reached, while at the same
time the calling code suppreses sending of rows.

Proposed solution:
Dedicated clear method for query initialization (to ensure no garbage
remains between executions).
Dedicated set_unlimited that only alters the limit value.