Remove the code nothing in production calls (closes #312)

Service.Warm, the ImageCache interface, both GenerateSignedURL methods,
signature.ParseParams, imagerequest.ParseImagePath, four symbols of
internal/magic, the image processor's lists of supported formats,
IsEmpty and Count of the allowlist, ssrfSafeDialer, CacheStale,
ErrCacheMiss, ErrNegativeCache, ImageResponse.LastModified and the Load
methods of ContentStorage and VariantStorage had no caller outside
tests. They are gone, with the tests that tested only them.

The handlers tests sign their image URLs with generateSignedURL, a test
helper that builds the URL as the removed generator did. The SSRF dialer
tests call dialSSRFSafe with no operator blocklist, which is all
ssrfSafeDialer did. The tests of ParseImagePath and ContentStorage.Load
call ParseImageURL and LoadWithSize instead. MIMEToImageFormat,
ContentStorage.Store and CleanExpired stay, as pixa calls them.

Model: opus-5-5

internal/handlers/request_id_internal_test.go

@@ -17,11 +17,13 @@ import (
 )
 
 // signedPhotoURL returns a signed /v1/image/ URL, valid for a minute, for the
-// JPEG at photoPath on signedHost at 50x50, made with h's image service.
-func signedPhotoURL(t *testing.T, h *Handlers) string {
+// JPEG at photoPath on signedHost at 50x50. It is signed with testSigningKey,
+// the key of the image service newSignedHostServer builds. It takes a Handlers
+// it does not use, so that it has the type of encPhotoURL.
+func signedPhotoURL(t *testing.T, _ *Handlers) string {
 	t.Helper()
 
-	signedURL, err := h.imgSvc.GenerateSignedURL("", &imagerequest.Request{
+	return generateSignedURL(testSigningKey, &imagerequest.Request{
 		Source: imagerequest.Source{
 			Host: signedHost,
 			Path: photoPath,
@@ -31,11 +33,6 @@ func signedPhotoURL(t *testing.T, h *Handlers) string {
 			Format: imagerequest.FormatJPEG,
 		},
 	}, time.Minute)
-	if err != nil {
-		t.Fatalf("GenerateSignedURL() error = %v", err)
-	}
-
-	return signedURL
 }
 
 // encPhotoURL returns an encrypted /v1/e/ URL, which never expires, for the