MDEV-19544 Remove innodb_locks_unsafe_for_binlog

The transaction isolation levels READ COMMITTED and READ UNCOMMITTED
should behave similarly to the old deprecated setting
innodb_locks_unsafe_for_binlog=1, that is, avoid acquiring gap locks.

row_search_mvcc(): Reduce the scope of some variables, and clean up
the initialization and use of the variable set_also_gap_locks.

mysql-test/suite/innodb/r/innodb_lock_wait_timeout_1.result

@@ -35,7 +35,6 @@ DROP TABLE t2, t1;
 # Bug#46539 Various crashes on INSERT IGNORE SELECT + SELECT
 #           FOR UPDATE
 #
-drop table if exists t1;
 create table t1 (a int primary key auto_increment,
 b int, index(b)) engine=innodb;
 insert into t1 (b) values (1), (2), (3), (4), (5), (6), (7), (8), (9), (10);
@@ -84,19 +83,12 @@ drop table t1, t2;
 #
 # Bug#41756 Strange error messages about locks from InnoDB
 #
-drop table if exists t1;
-# In the default transaction isolation mode, and/or with
-# innodb_locks_unsafe_for_binlog=OFF, handler::unlock_row()
-# in InnoDB does nothing.
+# In the default transaction isolation mode,
+# handler::unlock_row() in InnoDB does nothing.
 # Thus in order to reproduce the condition that led to the
 # warning, one needs to relax isolation by either
 # setting a weaker tx_isolation value, or by turning on
 # the unsafe replication switch.
-# For testing purposes, choose to tweak the isolation level,
-# since it's settable at runtime, unlike
-# innodb_locks_unsafe_for_binlog, which is
-# only a command-line switch.
-#
 set @@session.tx_isolation="read-committed";
 # Prepare data. We need a table with a unique index,
 # for join_read_key to be used. The other column