Remove migrate - Unnecessary complexity without current known use cases

[romeara]

Datastore.migrate() was added originally to cover a possible data-migration use case. However, this case can be addressed in other ways, and the inclusion of this method without strong supporting use-cases at this time is introducing unjustified overhead

[codecov[bot]]

Codecov Report

Merging #39 into master will decrease coverage by 1.54%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #39      +/-   ##
============================================
- Coverage     87.02%   85.47%   -1.55%     
+ Complexity      108      103       -5     
============================================
  Files            18       18              
  Lines           370      365       -5     
  Branches         34       33       -1     
============================================
- Hits            322      312      -10     
- Misses           34       38       +4     
- Partials         14       15       +1

Impacted Files	Coverage Δ	Complexity Δ
...va/com/coronaide/core/datastore/JsonDatastore.java	100% <ø> (ø)	4 <0> (-1)	:arrow_down:
.../coronaide/core/service/impl/DatastoreService.java	79.27% <100%> (-4.8%)	16 <0> (-4)

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