Allow --install-dir to be specified together with --user-install

The combination of `install-dir` and `--user-install` used to be
disabled for no good reason. This even makes problem on Linux
distributions such as Fedora, where `--user-install` is set by default
via operating_system.rb.

The `--install-dir` is already prefered over the `--user-install` by
the implementation, therefore just drop the check.

lib/rubygems/commands/install_command.rb

@@ -136,13 +136,6 @@ def usage # :nodoc:
     "#{program_name} [options] GEMNAME [GEMNAME ...] -- --build-flags"
   end
 
-  def check_install_dir # :nodoc:
-    if options[:install_dir] && options[:user_install]
-      alert_error "Use --install-dir or --user-install but not both"
-      terminate_interaction 1
-    end
-  end
-
   def check_version # :nodoc:
     if options[:version] != Gem::Requirement.default &&
        get_all_gem_names.size > 1
@@ -162,7 +155,6 @@ def execute
 
     ENV.delete "GEM_PATH" if options[:install_dir].nil?
 
-    check_install_dir
     check_version
 
     load_hooks

test/rubygems/test_gem_commands_install_command.rb

@@ -436,21 +436,6 @@ def test_execute_nonexistent_with_dashes
     assert_equal expected, output
   end
 
-  def test_execute_conflicting_install_options
-    @cmd.options[:user_install] = true
-    @cmd.options[:install_dir] = "whatever"
-
-    use_ui @ui do
-      assert_raise Gem::MockGemUi::TermError do
-        @cmd.execute
-      end
-    end
-
-    expected = "ERROR:  Use --install-dir or --user-install but not both\n"
-
-    assert_equal expected, @ui.error
-  end
-
   def test_execute_prerelease_skipped_when_no_flag_set
     spec_fetcher do |fetcher|
       fetcher.gem "a", 1