[rubygems/rubygems] Remove unused variable

https://github.com/rubygems/rubygems/commit/b31f33d55c

test/rubygems/test_gem_dependency_installer.rb

@@ -849,7 +849,6 @@ class TestGemDependencyInstaller < Gem::TestCase
     Gem.clear_paths
 
     Gem.done_installing do |installer, specs|
-      done_installing_ran = true
       refute_nil installer
       assert_equal [@b1], specs
     end