Fix display of relationship 0

Relation "duplicate of" has id value 0, and is being converted to "any"
in the filter box.
The code removed here does not make sense now as filters have a better
default and validation.

Fixes: #21867

core/filter_form_api.php

@@ -1588,9 +1588,6 @@ function print_filter_relationship_type( array $p_filter = null ) {
 		$p_filter = $g_filter;
 	}
 	$c_reltype_value = $p_filter[FILTER_PROPERTY_RELATIONSHIP_TYPE];
-	if( !$c_reltype_value ) {
-		$c_reltype_value = -1;
-	}
 	relationship_list_box( $c_reltype_value, 'relationship_type', true, true, "input-xs" );
 	echo '<input class="input-xs" type="text" name="', FILTER_PROPERTY_RELATIONSHIP_BUG, '" size="5" maxlength="10" value="', $p_filter[FILTER_PROPERTY_RELATIONSHIP_BUG], '" />';
 }