Fix deletion of default category on MSSQL/Oracle (take 2)

Use new config_is_defined() API to check whether the category to delete
is defined as default_category_for_moves, instead of executing a SQL
statement against the config table.

This prevents a type mismatch error on some RDBMS:

- Oracle (ORA-00932: inconsistent data types: expected -, found CLOB)
- MSSQL (Operand type clash: text is incompatible with int.)
- PostgreSQL (unconfirmed)

Fixes #16384

manage_proj_cat_delete.php

@@ -67,9 +67,9 @@
 
 # Protect the 'default category for moves' from deletion
 $t_default_category_id = config_get( 'default_category_for_moves', /* default */ null, ALL_USERS, ALL_PROJECTS );
-$t_query = 'SELECT count(config_id) FROM {config} WHERE config_id = ' . db_param() . ' AND value = ' . db_param();
-$t_default_cat_count = db_result( db_query( $t_query, array( 'default_category_for_moves', (string)$f_category_id ) ) );
-if( $t_default_cat_count > 0 || $f_category_id == $t_default_category_id ) {
+if(    $f_category_id == $t_default_category_id
+	|| config_is_defined( 'default_category_for_moves', $f_category_id )
+) {
 	trigger_error( ERROR_CATEGORY_CANNOT_DELETE_DEFAULT, ERROR );
 }