errcode: remove unused error codes

[shashankram]

Description:
A lot of unused error paths were cleaned up as a
part of refactoring efforts. This change removes
unused error codes.

Affected area:

Functional Area
Control Plane	[X]

Please answer the following questions with yes/no.

1. Does this change contain code from or inspired by another project? no

   • Did you notify the maintainers and provide attribution?

2. Is this a breaking change? no

[codecov-commenter]

Codecov Report

Merging #4077 (e922129) into main (0288571) will decrease coverage by 0.12%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #4077      +/-   ##
==========================================
- Coverage   69.04%   68.92%   -0.13%     
==========================================
  Files         205      205              
  Lines       11247    11249       +2     
==========================================
- Hits         7766     7753      -13     
- Misses       3429     3444      +15     
  Partials       52       52              

Flag	Coverage Δ
unittests	68.92% <0.00%> (-0.13%)	⬇️

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

Impacted Files	Coverage Δ
pkg/envoy/eds/response.go	47.69% <0.00%> (-1.52%)	⬇️
pkg/errcode/errcode.go	100.00% <ø> (ø)
pkg/crdconversion/crdconversion.go	59.78% <0.00%> (-15.22%)	⬇️
pkg/certificate/rotor/rotor.go	87.50% <0.00%> (+3.12%)	⬆️

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