Remove unused Subject field from CodeSigningCertificate

[nsmith5]

Summary

Removes the Subject field from CodeSigningCertificate and CodeSigningPreCertificate as the field is unused in the rest
of the code base. This change helps to remove the usage of challenges.ChallengeResult in general as we move to the newer
identity issuer abstraction.

Bonus: Check out that git commit signature! Signed by Fulcio :D

Release Note

NONE

[codecov-commenter]

Codecov Report

Merging #568 (579494a) into main (4ebe287) will increase coverage by 0.30%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
+ Coverage   43.17%   43.47%   +0.30%     
==========================================
  Files          20       20              
  Lines        1473     1472       -1     
==========================================
+ Hits          636      640       +4     
+ Misses        766      762       -4     
+ Partials       71       70       -1     

Impacted Files	Coverage Δ
pkg/ca/googleca/v1/googleca.go	50.43% <0.00%> (ø)
pkg/ca/x509ca/common.go	34.34% <0.00%> (ø)
pkg/ca/intermediateca/intermediateca.go	72.30% <66.66%> (-0.22%)	⬇️
pkg/ca/fileca/watch.go	100.00% <0.00%> (+50.00%)	⬆️

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