certificate: Delete unused functions

[draychev]

This PR deletes unused functions from pkg/certificate:

• LoadPrivateKeyFromFile
• LoadCertificateFromFile
• LoadCA

With that we can remove the tests for these functions and the pem files with cert and key - these no longer need to be files.

Created Cert and Key in the test fixtures module so these can be reused.

---

Affected area:

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?

[codecov-io]

Codecov Report

Merging #2305 (33e4ab4) into main (d8697f8) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2305      +/-   ##
==========================================
- Coverage   57.32%   57.28%   -0.04%     
==========================================
  Files         150      149       -1     
  Lines        6716     6677      -39     
==========================================
- Hits         3850     3825      -25     
+ Misses       2861     2847      -14     
  Partials        5        5              

Impacted Files	Coverage Δ
pkg/certificate/providers/tresor/certificate.go	85.71% <ø> (+7.58%)	⬆️
pkg/certificate/types.go	0.00% <ø> (ø)
pkg/envoy/rds/response.go	29.57% <0.00%> (-0.86%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d8697f8...33e4ab4. Read the comment docs.