ci: remove IgorMinar from reviewers list for pullapprove fallback group (#36456)

Historically we have had a pullapprove group `fallback` which acted as
a catch all for files which did not match any other groups.  This
group assigned reviews to IgorMinar, however it was not apparent that
this group was assigned.  This change removes this assignment.  This
group as active should always coincide with failures of the pullapprove
verification script. We continue to have this group as a secondary test
ensuring all files in the repo are captured by the pullapprove config.

PR Close #36456

dev-infra/pullapprove/parse-yaml.ts

@@ -9,7 +9,7 @@ import {parse as parseYaml} from 'yaml';
 
 export interface PullApproveGroupConfig {
   conditions?: string[];
-  reviewers: {
+  reviewers?: {
     users: string[],
     teams?: string[],
   }|{