Revert "reinstalls gem if gem dir is missing but spec exists"

It turns out that this has some pretty extensive repurcussions, and
makes it impossible to use Bundler in some specific circumstances, like
with gems that ship with Ruby, and with gems that are installed by distro
packages. It also breaks the `bundle pack` command, because the gems
that are cached by the pack command are not found by this check.

See also:

#2854
#2780

This reverts commit bd0243e.

lib/bundler/source/rubygems.rb

@@ -4,7 +4,6 @@
 
 module Bundler
   module Source
-    # TODO: Refactor this class
     class Rubygems
       API_REQUEST_LIMIT = 100 # threshold for switching back to the modern index instead of fetching every spec
 
@@ -68,7 +67,7 @@ def specs
       end
 
       def install(spec)
-        if installed_specs[spec].any? && gem_dir_exists?(spec)
+        if installed_specs[spec].any?
           return ["Using #{spec.name} (#{spec.version})", nil]
         end
 
@@ -277,16 +276,6 @@ def remote_specs
         end
       end
 
-      def gem_dir_exists?(spec)
-        return true if spec.name == "bundler"
-        # Ruby 2 default gems
-        return true if spec.loaded_from.include?("specifications/default/")
-        # Ruby 1.9 default gems
-        return true if spec.summary =~ /is bundled with Ruby/
-
-        File.directory?(spec.full_gem_path)
-      end
     end
-
   end
 end

spec/install/gems/simple_case_spec.rb

@@ -337,22 +337,6 @@
       G
       expect(exitstatus).to eq(0)
     end
-
-    it "reinstalls the gem if the gem dir is missing but the specification file exists" do
-      gemfile(<<-G)
-        source "file://#{gem_repo1}"
-
-        gem 'foo'
-      G
-
-      bundle "install --path vendor/bundle"
-
-      FileUtils.rm_rf(vendored_gems('gems/foo-1.0'))
-
-      bundle "install"
-
-      expect(vendored_gems('gems/foo-1.0')).to exist
-    end
   end
 
   describe "when Bundler root contains regex chars" do