[7.0] Drop flowUsesCommas option from babel-generator

[cnguy]

Q	A
Patch: Bug Fix?	no
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	yes?
Spec Compliancy?	yes?
Tests Added/Pass?	yes
Fixed Tickets	#5120
License	MIT
Doc PR	no
Dependency Changes	no

• remove the option for flowUsesCommas in the readme, code, tests
• just make the default commas (no more semi if its the enforced style at fb already anyway)

In addition to the above, I also deleted packages/babel-generator/test/fixtures/flowUsesCommas/options.json because it only had one option (flowCommaSeparator).

[mention-bot]

@ChauTNguyen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @sampepose, @bhosmer and @DrewML to be potential reviewers.

[codecov-io]

Current coverage is 89.20% (diff: 100%)

Merging #5123 into 7.0 will increase coverage by <.01%

@@                7.0      #5123   diff @@
==========================================
  Files           203        203          
  Lines          9819       9820     +1   
  Methods        1071       1071          
  Messages          0          0          
  Branches       2615       2615          
==========================================
+ Hits           8759       8760     +1   
  Misses         1060       1060          
  Partials          0          0          

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[loganfsmyth]

Seems like a breaking change to me, so probably a good candidate for 7.x

[loganfsmyth]

Oh this is for #5120 so yeah 7.x. Going to move your PR to be based on the 7.0 branch.

[hzoo]

Nice work @ChauTNguyen!! (merged to 7.0 branch)