Update dialog to not provide plugin-dir path to mysqltest

plugin-dir is provided via --defaults-file=<config-file> already part of
the exec call.

mysql-test/suite/plugins/t/dialog.test

@@ -15,8 +15,6 @@ if (!$DIALOG_EXAMPLES_SO) {
 eval install plugin three_attempts soname '$DIALOG_EXAMPLES_SO';
 create user test_dialog identified via three_attempts using 'SECRET';
 
-let $plugindir=`SELECT @@global.plugin_dir`;
-
 --write_file $MYSQLTEST_VARDIR/tmp/dialog_good.txt
 foo
 1234
@@ -34,19 +32,19 @@ EOF
 --echo #
 --echo # -pSECRET is picked up, no questions asked.
 --echo #
---exec echo "select user(), current_user();"|$MYSQL_TEST -u test_dialog -pSECRET --plugin-dir=$plugindir
+--exec echo "select user(), current_user();"|$MYSQL_TEST -u test_dialog -pSECRET
 
 --echo #
 --echo # without -p. up to three questions are asked on the stdin.
 --echo # athentication is successful, the correct pasword is on the third line
 --echo #
---exec $MYSQL_TEST -u test_dialog --plugin-dir=$plugindir < $MYSQLTEST_VARDIR/tmp/dialog_good.txt
+--exec $MYSQL_TEST -u test_dialog < $MYSQLTEST_VARDIR/tmp/dialog_good.txt
 
 --echo #
 --echo # athentication is unsuccessful, first three lines are all wrong
 --echo #
 --error 1
---exec $MYSQL_TEST -u test_dialog --plugin-dir=$plugindir < $MYSQLTEST_VARDIR/tmp/dialog_bad.txt
+--exec $MYSQL_TEST -u test_dialog < $MYSQLTEST_VARDIR/tmp/dialog_bad.txt
 
 --remove_file $MYSQLTEST_VARDIR/tmp/dialog_good.txt
 --remove_file $MYSQLTEST_VARDIR/tmp/dialog_bad.txt