remove trillian_log_sequencer_api.proto

[pphaneuf]

There are no methods in that service.

This is part of #2245.

[codecov]

Codecov Report

Merging #2337 (a74f478) into master (0aa1890) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2337      +/-   ##
==========================================
- Coverage   66.00%   65.96%   -0.04%     
==========================================
  Files         105      105              
  Lines        7745     7745              
==========================================
- Hits         5112     5109       -3     
- Misses       2098     2100       +2     
- Partials      535      536       +1     

Impacted Files	Coverage Δ
client/log_client.go	70.93% <0.00%> (-2.91%)	⬇️
log/operation_manager.go	89.41% <0.00%> (+1.05%)	⬆️

---

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 0aa1890...a74f478. Read the comment docs.