remove unused cruft from reconciler split

[vaikas]

Proposed Changes

• When we split the broker reconciler into broker / trigger, some cruft was left over. My bad, removing...

Release Note

- 🧽 Update or clean up current behavior
Just clean up some unused fields from the mtbroker reconciler struct.

Docs

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/mtbroker/controller.go	90.5%	88.9%	-1.6

[codecov]

Codecov Report

Merging #4318 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4318      +/-   ##
==========================================
- Coverage   80.36%   80.35%   -0.01%     
==========================================
  Files         288      288              
  Lines        7910     7907       -3     
==========================================
- Hits         6357     6354       -3     
  Misses       1172     1172              
  Partials      381      381              

Impacted Files	Coverage Δ
pkg/reconciler/mtbroker/broker.go	78.44% <ø> (ø)
pkg/reconciler/mtbroker/controller.go	93.02% <ø> (-0.46%)	⬇️

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[pierDipi]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pierDipi, vaikas

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