Remove --plugin-dir option from mysql_client_test test case.

This gives the wrong path when testing installed server, and we
set the correct path in my.cnf anyway.

mysql-test/t/mysql_client_test.test

@@ -14,7 +14,7 @@ SET @old_slow_query_log= @@global.slow_query_log;
 # var/log/mysql_client_test.trace
 
 --exec echo "$MYSQL_CLIENT_TEST" > $MYSQLTEST_VARDIR/log/mysql_client_test.out.log 2>&1
---exec $MYSQL_CLIENT_TEST --getopt-ll-test=25600M --plugin-dir=$MYSQLTEST_VARDIR/plugins >> $MYSQLTEST_VARDIR/log/mysql_client_test.out.log 2>&1
+--exec $MYSQL_CLIENT_TEST --getopt-ll-test=25600M >> $MYSQLTEST_VARDIR/log/mysql_client_test.out.log 2>&1
 
 # End of 4.1 tests
 echo ok;