TT#37113 Use safe 'DROP USER' syntax

Replace unsafe 'DELETE FROM mysql.user' with recommended 'DROP USER'
statement to avoid problems with DROP/CREATE USER due to missing
'FLUSH PRIVILEGES' like:
Error: Cannot create grant temp user: Operation CREATE USER failed
for 'ngcp-sync-db'@'localhost' at /usr/sbin/ngcp-sync-grants line 322.

Execute 'FLUSH PRIVILEGES' before creating and after dropping of temp user
'ngcp-sync-db'.

Change-Id: I49c29b6c39353d4a47f086851a915af1469ebcdd

sbin/ngcp-sync-grants

@@ -261,10 +261,6 @@ sub apply_copy_grants {
 sub apply_drop_users {
     my $forced_user = shift; # to drop a specific user
 
-    my $ch = $dbh->prepare(<<SQL)
-DELETE FROM mysql.user WHERE User = ? AND Host LIKE ?
-SQL
-        or die "Cannot prepare: ".$DBI::errstr;
     my $ch_sel = $dbh->prepare(<<SQL)
 SELECT Host FROM mysql.user WHERE User = ? AND Host LIKE ?
 SQL
@@ -285,7 +281,7 @@ SQL
             $ch_sel->execute($user, $host_rx)
                 or die "Cannot select user $user -- $host_rx: ".$DBI::errstr;
             while (my ($host) = $ch_sel->fetchrow_array) {
-                $ch->execute($user, $host_rx)
+                $dbh->do("DROP USER '$user'\@'$host_rx'")
                     or die "Cannot drop user $user -- $host_rx: ".$DBI::errstr;
                 log_info(sprintf "drop: %s\@%s", $user, $host);
                 $rc++;
@@ -293,7 +289,6 @@ SQL
         }
     }
 
-    $ch->finish;
     $ch_sel->finish;
 
     return $rc;
@@ -347,6 +342,7 @@ sub grants_helper {
     my $user = $TEMP_GRANT_USER;
     my $host = $TEMP_GRANT_HOST;
 
+    flush_privs();
     set_user_protected_password($user, $host);
     $dbh->do("REVOKE ALL PRIVILEGES, GRANT OPTION FROM '$user'\@'$host'");
 
@@ -378,6 +374,7 @@ sub grants_helper {
 
     $dbh->do("DROP USER '$user'\@'$host'");
     die "Cannot drop grant temp user: ".$DBI::errstr if $DBI::err;
+    flush_privs();
 
     @$grants = ( map { $_->[0] } @$temp_grants );