filter_api: ensure that the free_text where clauses are always ANDed

Fixes #15573: One query can be issued via current Mantis interface to
take down site
Revert "filter_api: ensure that the free_text where clauses are always ANDed"

This reverts commit 543ba012999c20310a2208bd8e5f7f88efe392ce.
Fix filter api issue with 'any condition' and text search

A filter combining some criteria and a text search with 'any condition'
results in a cartesian product, which has the potential to bring down
the site as the RDBMS eats up all available resources.

The root cause of this behavior is joining the bug_text table with a
from clause and setting the join's criteria in the query's where clause,
without taking consideration the operator's precedence (AND/OR).

This commit resolves the problem by using a JOIN clause instead, which
makes the query cleaner.

Fixes #15573
Filter api: systematic use JOIN when building SQL

Do not join tables using the where clause, for better readability and
avoiding risk of issues with operator precedence and 'any condition'
filtering mode.

This commit also removes an unnecessary LEFT JOIN between the bugnote
and bugnote_text tables; since this is a strict 1:1 relationship, an
inner join is sufficient and yields better performance.