Fix the partially quoted index name for MariaDB/MySQL

Previously the index name was generated with quoted table name and a
number suffix like "`attachments`1", which MariaDB/MySQL does not
support.

As index names(table name + number suffix) probably will never be
reserved words, this commit simply removes the quote of the table name
part.

This is initially to fix upgrade steps that create indexes via "indexes"
file.

lib/RT/Handle.pm

@@ -2249,8 +2249,13 @@ sub CreateIndex {
     my $self = shift;
     my %args = ( Table => undef, Name => undef, Columns => [], CaseInsensitive => {}, @_ );
 
+    my $quoted_table; # Quoted table only for mysql.
     if (RT->Config->Get('DatabaseType') eq 'mysql') {
-        $args{'Table'} = $self->QuoteName( $self->_CanonicTableNameMysql( $args{'Table'} ));
+        $args{'Table'} = $self->_CanonicTableNameMysql( $args{'Table'} );
+        $quoted_table = $self->QuoteName($args{'Table'});
+    }
+    else {
+        $quoted_table = $args{'Table'};
     }
 
     my $name = $args{'Name'};
@@ -2270,9 +2275,10 @@ sub CreateIndex {
         }
     }
 
+
     my $sql = "CREATE"
         . ($args{'Unique'}? ' UNIQUE' : '')
-        ." INDEX $name ON $args{'Table'}"
+        ." INDEX $name ON $quoted_table"
         ."(". join( ', ', @columns ) .")"
     ;