Uninstall rubocop-thread_safety temporarily

It breaks the build because it requires rubocop 0.47, which
we have not upgraded to yet.

I'm not sure how the addition of rubocop-thread_safety passed CI.

I want to try it out, but don't want to upgrade rubocop right now.

.rubocop.yml

@@ -1,6 +1,5 @@
 require:
   - rubocop-rspec
-  - rubocop-thread_safety
 
 inherit_from: .rubocop_todo.yml
 
@@ -109,11 +108,3 @@ Style/StringLiterals:
 # because it makes the code harder to edit, and makes lines unnecessarily long.
 Style/SpaceAroundOperators:
   AllowForAlignment: false
-
-ThreadSafety/NewThread:
-  Exclude:
-    - 'test/functional/thread_safety_test.rb'
-
-ThreadSafety/InstanceVariableInClassMethod:
-  Exclude:
-    - 'lib/paper_trail.rb'

paper_trail.gemspec

@@ -43,7 +43,6 @@ has been destroyed.
   s.add_development_dependency "pry-nav", "~> 0.2.4"
   s.add_development_dependency "rubocop", "~> 0.41.1"
   s.add_development_dependency "rubocop-rspec", "~> 1.5.1"
-  s.add_development_dependency "rubocop-thread_safety", "~> 0.3.1"
   s.add_development_dependency "timecop", "~> 0.8.0"
   s.add_development_dependency "sqlite3", "~> 1.2"
   s.add_development_dependency "pg", "~> 0.19.0"