Remove GetSequencedLeafCount from the storage interface.

[pphaneuf]

Part of #2245.

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

[codecov]

Codecov Report

Merging #2439 (69136d4) into master (ba7a41b) will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2439      +/-   ##
==========================================
+ Coverage   65.89%   66.03%   +0.14%     
==========================================
  Files         104      104              
  Lines        7240     7223      -17     
==========================================
- Hits         4771     4770       -1     
+ Misses       1953     1939      -14     
+ Partials      516      514       -2     

Impacted Files	Coverage Δ
storage/cloudspanner/log_storage.go	56.11% <ø> (+0.43%)	⬆️
storage/memory/log_storage.go	3.77% <ø> (+0.13%)	⬆️
storage/mysql/log_storage.go	65.95% <ø> (-0.09%)	⬇️
client/log_client.go	77.48% <0.00%> (+2.64%)	⬆️

---

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 ba7a41b...69136d4. Read the comment docs.