file ref: Remove build files that refer to it when removed.

`PBXReferenceProxy` and `PBXGroup`  might have build files that refer
to it, and as mentioned in the previous commit, they don't have any
meaning without it. Therefore everything that can be a file reference
removes the build files that refer to it.

lib/xcodeproj/project/object/group.rb

@@ -448,6 +448,25 @@ def sort(options = nil)
             result
           end
         end
+
+        # @return [Array<PBXBuildFile>] the build files associated with the
+        #         current reference proxy.
+        #
+        def build_files
+          referrers.grep(PBXBuildFile)
+        end
+
+        # In addition to removing the reference proxy, this will also remove any
+        # items related to this reference.
+        #
+        # @see AbstractObject#remove_from_project
+        #
+        # @return [void]
+        #
+        def remove_from_project
+          build_files.each(&:remove_from_project)
+          super
+        end
       end
 
       #-----------------------------------------------------------------------#

lib/xcodeproj/project/object/reference_proxy.rb

@@ -61,6 +61,25 @@ def display_name
           return path if path
           super
         end
+
+        # @return [Array<PBXBuildFile>] the build files associated with the
+        #         current reference proxy.
+        #
+        def build_files
+          referrers.grep(PBXBuildFile)
+        end
+
+        # In addition to removing the reference proxy, this will also remove any
+        # items related to this reference.
+        #
+        # @see AbstractObject#remove_from_project
+        #
+        # @return [void]
+        #
+        def remove_from_project
+          build_files.each(&:remove_from_project)
+          super
+        end
       end
     end
   end

spec/project/object/group_spec.rb

@@ -345,6 +345,15 @@ module ProjectSpecs
       end
     end
 
+    it 'removes the build files when removing the group' do
+      @target = @project.new_target(:static_library, 'Pods', :ios)
+      build_file = @project.new(Xcodeproj::Project::PBXBuildFile)
+      build_file.file_ref = @group
+      @target.build_phases[0].files << build_file
+
+      @group.remove_from_project
+      @target.build_phases[0].files.should.be.empty
+    end
     #-------------------------------------------------------------------------#
   end
 end

spec/project/object/reference_proxy_spec.rb

@@ -24,5 +24,13 @@ module ProjectSpecs
       @proxy.path = 'Path/To/Proxy'
       @proxy.display_name.should == 'Path/To/Proxy'
     end
+
+    it 'removes the build files when removing the reference proxy' do
+      @target = @project.new_target(:static_library, 'Pods', :ios)
+      @target.build_phases[0].add_file_reference(@proxy)
+
+      @proxy.remove_from_project
+      @target.build_phases[0].files.should.be.empty
+    end
   end
 end