fix: remove unused StringConstructor from return type

Address review feedback from @michael-s-molina: Since the function
no longer returns String, remove StringConstructor from the return
type and update the misleading test description.

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

superset-frontend/plugins/plugin-chart-echarts/src/utils/formatters.ts

@@ -88,7 +88,7 @@ export function getTooltipTimeFormatter(
 
 export function getXAxisFormatter(
   format?: string,
-): TimeFormatter | StringConstructor | undefined {
+): TimeFormatter | undefined {
   if (format === SMART_DATE_ID) {
     return getSmartDateFormatter();
   }