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

Service.Warm, the ImageCache interface, both GenerateSignedURL methods,
signature.ParseParams, three functions of internal/magic, the image
processor's lists of supported formats, IsEmpty and Count of the
allowlist, ssrfSafeDialer, CacheStale, ErrCacheMiss, ErrNegativeCache,
ImageResponse.LastModified and VariantStorage.Load 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. ParseImageURL, MIMEToImageFormat, ContentStorage.Store
and CleanExpired stay, as pixa calls them.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -322,30 +322,6 @@ const (
 	mimeJXL  = "image/jxl"
 )
 
-// SupportedInputFormats returns MIME types this processor can read.
-func (p *ImageProcessor) SupportedInputFormats() []string {
-	return []string{
-		mimeJPEG,
-		mimePNG,
-		mimeGIF,
-		mimeWebP,
-		mimeAVIF,
-		mimeJXL,
-	}
-}
-
-// SupportedOutputFormats returns formats this processor can write.
-func (p *ImageProcessor) SupportedOutputFormats() []imagerequest.Format {
-	return []imagerequest.Format{
-		imagerequest.FormatJPEG,
-		imagerequest.FormatPNG,
-		imagerequest.FormatGIF,
-		imagerequest.FormatWebP,
-		imagerequest.FormatAVIF,
-		imagerequest.FormatJXL,
-	}
-}
-
 // FormatToMIME converts a Format to its MIME type string.
 func FormatToMIME(format imagerequest.Format) string {
 	switch format {