[WIP] delete unused backend fields

[ElvinEfendi]

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[ElvinEfendi]

secure-verify-ca-secret

This was added at #716. Maybe at the time it was working and over time someone mistakenly removed its usage, but looking at the code that annotation is not being used. Also it's been deprecated as part #2871, and we should encapsulate it in Location type of rather than Backend just like BackendProtocol annotation.

[codecov-io]

Codecov Report

Merging #2920 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2920      +/-   ##
==========================================
- Coverage   47.56%   47.54%   -0.03%     
==========================================
  Files          77       77              
  Lines        5531     5502      -29     
==========================================
- Hits         2631     2616      -15     
+ Misses       2560     2550      -10     
+ Partials      340      336       -4

Impacted Files	Coverage Δ
internal/ingress/types.go	0% <ø> (ø)	⬆️
internal/ingress/types_equals.go	13.35% <ø> (+0.18%)	⬆️
internal/ingress/zz_generated.deepcopy.go	0% <ø> (ø)	⬆️
internal/ingress/controller/nginx.go	11.28% <ø> (-0.4%)	⬇️
...nternal/ingress/annotations/secureupstream/main.go	100% <ø> (+18.18%)	⬆️
internal/ingress/controller/controller.go	2.22% <0%> (+0.02%)	⬆️
internal/ingress/controller/template/template.go	64.83% <100%> (ø)	⬆️
internal/ingress/annotations/annotations.go	77.77% <0%> (-3.18%)	⬇️

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[aledbf]

@ElvinEfendi friendly ping