[APM] Remove error.id in getErrorGroupMainStatistics query as it'…

…s not used (elastic#210613)

## Summary

Closes elastic#210610

This PR removes `error.id` field from being queried at
`getErrorGroupMainStatistics`, as it was not being used, as it was
required. If we didn't have this field, the endpoint call would crash.

(cherry picked from commit 46cd29e)

x-pack/solutions/observability/plugins/apm/server/routes/errors/get_error_groups/get_error_group_main_statistics.ts

@@ -17,7 +17,6 @@ import {
   ERROR_EXC_TYPE,
   ERROR_GROUP_ID,
   ERROR_GROUP_NAME,
-  ERROR_ID,
   ERROR_LOG_MESSAGE,
   SERVICE_NAME,
   TRACE_ID,
@@ -97,7 +96,7 @@ export async function getErrorGroupMainStatistics({
       ]
     : [];
 
-  const requiredFields = asMutableArray([AT_TIMESTAMP, ERROR_GROUP_ID, ERROR_ID] as const);
+  const requiredFields = asMutableArray([AT_TIMESTAMP, ERROR_GROUP_ID] as const);
 
   const optionalFields = asMutableArray([
     TRACE_ID,