Remove codegen from travis file

[algobarb]

Summary

Codegen verification step is now being run by CircleCI, so it's no longer necessary to run it on Travis, especially since we recently got a toomanyrequests: You have reached your pull rate limit. docker error. We want to reduce unnecessary docker requests.

Test Plan

Will check the PR tests to make sure codegen is no longer run on Travis tests.

[codecov-commenter]

Codecov Report

Merging #2768 (d08150e) into master (d7e9240) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2768   +/-   ##
=======================================
  Coverage   47.08%   47.08%           
=======================================
  Files         349      349           
  Lines       56348    56348           
=======================================
+ Hits        26529    26530    +1     
  Misses      26843    26843           
+ Partials     2976     2975    -1     

Impacted Files	Coverage Δ
crypto/merkletrie/trie.go	66.42% <0.00%> (-2.19%)	⬇️
crypto/merkletrie/node.go	91.62% <0.00%> (-1.87%)	⬇️
cmd/tealdbg/debugger.go	72.86% <0.00%> (-1.01%)	⬇️
network/requestTracker.go	70.25% <0.00%> (-0.87%)	⬇️
network/wsPeer.go	74.65% <0.00%> (+0.27%)	⬆️
ledger/acctupdates.go	62.63% <0.00%> (+0.50%)	⬆️
catchup/service.go	70.12% <0.00%> (+0.77%)	⬆️
ledger/blockqueue.go	82.18% <0.00%> (+1.14%)	⬆️

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[egieseke]

Looks good.