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/imgcache/storage.go

@@ -62,22 +62,6 @@ func (s *ContentStorage) Store(
 	return int64(len(data)), nil
 }
 
-// Load returns a reader for the content with the given key.
-func (s *ContentStorage) Load(key imagerequest.SourceKey) (io.ReadCloser, error) {
-	path := s.keyToPath(key)
-
-	f, err := os.Open(path) //nolint:gosec // path derived from source key
-	if err != nil {
-		if os.IsNotExist(err) {
-			return nil, ErrNotFound
-		}
-
-		return nil, fmt.Errorf("failed to open content: %w", err)
-	}
-
-	return f, nil
-}
-
 // LoadWithSize returns a reader and file size for the content with the given key.
 func (s *ContentStorage) LoadWithSize(
 	key imagerequest.SourceKey,
@@ -251,22 +235,6 @@ func (s *VariantStorage) Store(
 	return int64(len(data)), nil
 }
 
-// Load returns a reader for the content at the given key.
-func (s *VariantStorage) Load(key imagerequest.VariantKey) (io.ReadCloser, error) {
-	path := s.keyToPath(key)
-
-	f, err := os.Open(path) //nolint:gosec // path derived from cache key
-	if err != nil {
-		if os.IsNotExist(err) {
-			return nil, ErrNotFound
-		}
-
-		return nil, fmt.Errorf("failed to open content: %w", err)
-	}
-
-	return f, nil
-}
-
 // LoadWithContentType opens the content at the given key and returns a
 // reader, the file size and the content type, which is read from the
 // file's first bytes. A missing file is ErrNotFound. A file whose first