Cleanup Rakefile; remove unused rake tasks

We included rake tasks to build the gem, but out pipeline doesn't use
it. We always build with `gem build`.

Rakefile

@@ -1,14 +1,5 @@
-require 'rake'
-require 'rake/clean'
-require 'rubygems'
-require 'bundler/gem_tasks'
-require 'fileutils'
-require 'rspec/core'
 require 'rspec/core/rake_task'
 
-CLEAN.include('pkg/', 'tmp/')
-CLOBBER.include('Gemfile.lock')
-
 task default: [:spec]
 
 RSpec::Core::RakeTask.new(:spec) do |spec|