Remove unused `be_valid_commit` matcher

spec/support/matchers.rb

-RSpec::Matchers.define :be_valid_commit do
-  match do |actual|
-    actual &&
-      actual.id == ValidCommit::ID &&
-      actual.message == ValidCommit::MESSAGE &&
-      actual.author_name == ValidCommit::AUTHOR_FULL_NAME
-  end
-end
-
 def emulate_user(user)
   user = case user
          when :user then create(:user)