Remove issuance chain cache

[phbnf]

This was added to the CTFE to cache individual certs in the issuance chain. We don't need this anymore since the SCTFE won't serve issuance chain certs anymore, at least not with bucket styled storage system.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 34.99%. Comparing base (46ec9c2) to head (1332697).
Report is 46 commits behind head on main.

Files	Patch %	Lines
personalities/sctfe/ct_server_gcp/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   35.80%   34.99%   -0.81%     
==========================================
  Files          16       33      +17     
  Lines        1363     2706    +1343     
==========================================
+ Hits          488      947     +459     
- Misses        801     1652     +851     
- Partials       74      107      +33     

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