fix: Remove readiness tracker deadlock caused by duplicate syncs

[maxsmythe]

What this PR does / why we need it:

The following sync config would cause readiness tracker to hang:

apiVersion: config.gatekeeper.sh/v1alpha1
kind: Config
metadata:
  name: config
  namespace: gatekeeper-system
spec:
  sync:
    syncOnly:
    - group: ""
      kind: Namespace
      version: v1
    - group: networking.k8s.io
      kind: NetworkPolicy
      version: v1
    - group: networking.k8s.io
      kind: NetworkPolicy
      version: v1

this PR fixes that.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Patch coverage: 57.14% and no project coverage change.

Comparison is base (83ccdd3) 53.10% compared to head (4230479) 53.10%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2970   +/-   ##
=======================================
  Coverage   53.10%   53.10%           
=======================================
  Files         135      135           
  Lines       11806    11813    +7     
=======================================
+ Hits         6269     6273    +4     
- Misses       5051     5053    +2     
- Partials      486      487    +1     

Flag	Coverage Δ
unittests	53.10% <57.14%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
pkg/readiness/tracker_map.go	73.13% <25.00%> (-3.06%)	⬇️
pkg/readiness/object_tracker.go	84.26% <100.00%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

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[ritazh]

LGTM