cleanup unused certificates

[ElvinEfendi]

What this PR does / why we need it:

Currently when TLS section from an ingress is removed we don't cleanup corresponding certificate from Lua shared dict. This PR makes sure we clean them up. Notice that I'm still not cleaning the certificate itself and deleting only server. I can not delete certificate because relationship between certificates and servers is one to many.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1dc4d18). Click here to learn what that means.
The diff coverage is 64.28%.

@@            Coverage Diff            @@
##             master    #4594   +/-   ##
=========================================
  Coverage          ?   58.58%           
=========================================
  Files             ?       88           
  Lines             ?     6735           
  Branches          ?        0           
=========================================
  Hits              ?     3946           
  Misses            ?     2352           
  Partials          ?      437

Impacted Files	Coverage Δ
internal/ingress/controller/nginx.go	29.8% <64.28%> (ø)

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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