Remove unused ComputeDataCommitment from built-in market

[anorth]

Note this is based on master, but I could rebase on integration/direct-onboarding if desired.

[codecov-commenter]

Codecov Report

Merging #1354 (727811f) into master (ad4470f) will decrease coverage by 0.60%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1354      +/-   ##
==========================================
- Coverage   90.81%   90.21%   -0.60%     
==========================================
  Files         144      144              
  Lines       27346    27457     +111     
==========================================
- Hits        24834    24771      -63     
- Misses       2512     2686     +174     

Files Changed	Coverage Δ
actors/market/src/lib.rs	79.28% <ø> (-13.22%)	⬇️
actors/market/src/types.rs	62.22% <ø> (-34.66%)	⬇️
actors/miner/src/ext.rs	67.74% <ø> (+4.10%)	⬆️

... and 3 files with indirect coverage changes

[ZenGround0]

Since the market actor method passes most interesting functionality to the compute_data_commitment free function that's still in use it might be worth keeping some of these around in some form to test the free function.

[anorth]

I had checked the compute_data_commitment free function and it's too trivial to test, just a call through to runtime (which in these unit tests is a mock anyway).