chore: cleanup of leftover remnants of Timestamps in Snapshots

[BenWhitehead]

Related to #308

[codecov]

Codecov Report

Merging #326 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #326      +/-   ##
============================================
+ Coverage     72.08%   72.10%   +0.02%     
  Complexity      968      968              
============================================
  Files            60       60              
  Lines          5145     5138       -7     
  Branches        651      649       -2     
============================================
- Hits           3709     3705       -4     
+ Misses         1231     1229       -2     
+ Partials        205      204       -1     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/firestore/FirestoreOptions.java	36.36% <ø> (-1.25%)	7.00 <0.00> (ø)

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