aws-okta: remove gopath

[alebcay]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

Remove GOPATH as formula supports Go modules.

Relates to #47627.

[chenrui333]

02:03:27 ==> brew test aws-okta --verbose
02:03:30 ==> FAILED
02:03:30 Testing aws-okta
02:03:30 /usr/bin/sandbox-exec -f /private/tmp/homebrew20191209-33328-7ko5vs.sb ruby -W0 -I $LOAD_PATH -- /usr/local/Homebrew/Library/Homebrew/test.rb /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/aws-okta.rb --verbose
02:03:30 Error: aws-okta: failed
02:03:30 An exception occurred within a child process:
02:03:30 NoMethodError: undefined method `chomp' for nil:NilClass

[alebcay]

@BrewTestBot test this please.

[alebcay]

I am unable to reproduce the test failure locally on Catalina.

[chenrui333]

Works on my Catalina machine as well.

[chenrui333]

@BrewTestBot test this please

[chenrui333]

Still only failed on test step of catalina build. Go for the merge now.