http/response: get rid of the X-XSS-Protection header

It's useless at best, dangerous at worst, and shouldn't be used anymore
anywhere. See the following resources for details:

- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection
- https://chromestatus.com/feature/5021976655560704
- https://bugzilla.mozilla.org/show_bug.cgi?id=528661
- https://blogs.windows.com/windows-insider/2018/07/25/announcing-windows-10-insider-preview-build-17723-and-build-18204/

internal/http/response/builder.go

@@ -96,7 +96,6 @@ func (b *Builder) Write() {
 }
 
 func (b *Builder) writeHeaders() {
-	b.headers["X-XSS-Protection"] = "1; mode=block"
 	b.headers["X-Content-Type-Options"] = "nosniff"
 	b.headers["X-Frame-Options"] = "DENY"
 	b.headers["Referrer-Policy"] = "no-referrer"

internal/http/response/builder_test.go

@@ -28,7 +28,6 @@ func TestResponseHasCommonHeaders(t *testing.T) {
 	resp := w.Result()
 
 	headers := map[string]string{
-		"X-XSS-Protection":       "1; mode=block",
 		"X-Content-Type-Options": "nosniff",
 		"X-Frame-Options":        "DENY",
 	}