chore: remove danger

[ovasdi]

https://artsyproduct.atlassian.net/browse/PHIRE-82?focusedCommentId=58222

Before unsetting DANGER_GITHUB_API_TOKEN in this CircleCI project environment this PR removes Dangerfile and danger gem seeing as we no longer execute danger in CI.

PR that added danger.

[ovasdi]

Would it be preferable to re-enable this gem in CI for this project instead of removing it?

I tried invoking danger locally and I see the following:

➜  rosalind git:(main) bundle exec danger
Could not find the type of CI for Danger to run on.
➜  rosalind git:(main) danger

---------------------------------------------
Error loading the plugin `claide-plugins-0.9.2`.

NoMethodError - undefined method `activate' for #<Gem::StubSpecification:0x000000014a03a3e0>
Did you mean?  activated?
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command/plugin_manager.rb:93:in `safe_activate_and_require'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command/plugin_manager.rb:31:in `block in load_plugins'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command/plugin_manager.rb:30:in `map'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command/plugin_manager.rb:30:in `load_plugins'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command.rb:326:in `block in run'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command.rb:325:in `each'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command.rb:325:in `run'
...ruby/2.7.3/lib/ruby/gems/2.7.0/gems/danger-6.1.0/bin/danger:5:in `<top (required)>'
...ruby/2.7.3/bin/danger:25:in `load'
...ruby/2.7.3/bin/danger:25:in `<main>'
---------------------------------------------

Could not find the type of CI for Danger to run on.

I didn't dig deeper than that.