Use XAML binding for PR creation cancel button.

Author: grokys

src/GitHub.VisualStudio/UI/Views/PullRequestCreationView.xaml

@@ -298,7 +298,11 @@
                                 <ColumnDefinition Width="Auto" />
                             </Grid.ColumnDefinitions>
 
-                            <Button Name="cancelButton" HorizontalAlignment="Right" Content="Cancel" Grid.Column="0" Style="{StaticResource GitHubVsButton}"/>
+                            <Button Grid.Column="0"
+                                    HorizontalAlignment="Right"
+                                    Content="Cancel"
+                                    Style="{StaticResource GitHubVsButton}"
+                                    Command="{Binding CancelCommand}"/>
                             <Button HorizontalAlignment="Right" Margin="6,0,0,0" Content="Create pull request" Grid.Column="1" Style="{StaticResource GitHubVsPrimaryActionButton}" />
                         </Grid>
                     </StackPanel>

src/GitHub.VisualStudio/UI/Views/PullRequestCreationView.xaml.cs

@@ -21,7 +21,6 @@ public PullRequestCreationView()
 
             this.WhenActivated(d =>
             {
-                d(this.BindCommand(ViewModel, vm => vm.Cancel, v => v.cancelButton));
                 d(ViewModel.CancelCommand.Subscribe(_ => NotifyCancel()));
             });
         }