Remove now-unnecessary guard which checks for errors.

This is no longer necessary after the changes in @jbaxleyiii's
#2574 (and
@martijnwalraven's
#2416), which changed the
error reporting to be captured via the newly introduced
`didEncounterErrors`, rather than in `willResolveField` where it used to
happen.  Yay!

packages/apollo-engine-reporting/src/extension.ts

@@ -288,11 +288,6 @@ export class EngineReportingExtension<TContext = any>
   }
 
   public didEncounterErrors(errors: GraphQLError[]) {
-    // This life-cycle method is only invoked to capture errors.
-    if (!errors) {
-      return;
-    }
-
     errors.forEach(err => {
       // In terms of reporting, errors can be re-written by the user by
       // utilizing the `rewriteError` parameter.  This allows changing