Remove incorrect concat_ws to concat normalization

concat_ws should NOT be normalized to concat - ClickHouse preserves
the function name as concat_ws in EXPLAIN AST output.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

internal/explain/format.go

@@ -455,8 +455,6 @@ func NormalizeFunctionName(name string) string {
 		"trim":  "trimBoth",
 		"ltrim": "trimLeft",
 		"rtrim": "trimRight",
-		// CONCAT_WS is normalized to concat
-		"concat_ws": "concat",
 		// Position is normalized to lowercase
 		"position": "position",
 		// SUBSTRING is normalized to lowercase (but SUBSTR preserves case)

parser/testdata/03593_funcs_on_empty_string/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt91": true
-  }
-}
+{}