feat: Remove migration_shard_id check from AssignShardIdFn

[manitgupta]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 49.59%. Comparing base (87368b4) to head (4f24888). Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2467      +/-   ##
============================================
- Coverage     49.61%   49.59%   -0.02%     
- Complexity     4803     5153     +350     
============================================
  Files           941      941              
  Lines         57665    57658       -7     
  Branches       6235     6233       -2     
============================================
- Hits          28609    28597      -12     
- Misses        27011    27017       +6     
+ Partials       2045     2044       -1     

Components	Coverage Δ
spanner-templates	69.88% <100.00%> (-0.04%)	:arrow_down:
spanner-import-export	68.48% <ø> (-0.14%)	:arrow_down:
spanner-live-forward-migration	78.77% <ø> (ø)
spanner-live-reverse-replication	77.36% <100.00%> (+0.05%)	:arrow_up:
spanner-bulk-migration	87.87% <100.00%> (+0.01%)	:arrow_up:

Files with missing lines	Coverage Δ
...eport/v2/templates/transforms/AssignShardIdFn.java	79.00% <100.00%> (+1.59%)	:arrow_up:

... and 5 files with indirect coverage changes

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