Remove Ambiguous types.User.DirectoryId

[gdbelvin]

The DirectoryId in User is ambiguous because it is already implicitly defined by the public key of the directory, resulting in numerous checks to verify that these two values are in sync.

[codecov]

Codecov Report

Merging #1154 into master will decrease coverage by 0.52%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
- Coverage    65.1%   64.58%   -0.53%     
==========================================
  Files          41       41              
  Lines        3112     3103       -9     
==========================================
- Hits         2026     2004      -22     
- Misses        751      757       +6     
- Partials      335      342       +7

Impacted Files	Coverage Δ
core/integration/monitor_tests.go	74.62% <ø> (-1.09%)	⬇️
core/integration/client_tests.go	85.89% <ø> (-0.24%)	⬇️
core/client/client.go	66.03% <100%> (+0.82%)	⬆️
core/client/batch_client.go	62.26% <100%> (ø)	⬆️
core/client/get_and_verify.go	55.12% <50%> (ø)	⬆️
core/client/batch_get_and_verify.go	70.58% <75%> (ø)	⬆️
core/sequencer/trillian_client.go	56.36% <0%> (-5.46%)	⬇️
impl/sql/mutationstorage/mutations.go	68.42% <0%> (-5.27%)	⬇️
core/sequencer/server.go	68.97% <0%> (-3.27%)	⬇️
impl/sql/directory/storage.go	70.47% <0%> (-1.91%)	⬇️

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