Replace the MD5 hashing of images with xxHash

Note that we only use this for change detection.

The previous implementation invoked `MD5FromReaderFast` that created a MD5 has from 8 64 bytes chunks in the file, which is obviously very fast. The new implementation creates the hash from the entire file and ... seems to be even more effective:

```
name          old time/op    new time/op    delta
HashImage-10    9.45µs ±21%   10.89µs ± 1%     ~     (p=0.343 n=4+4)

name          old alloc/op   new alloc/op   delta
HashImage-10      144B ± 0%        8B ± 0%  -94.44%  (p=0.029 n=4+4)

name          old allocs/op  new allocs/op  delta
HashImage-10      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.029 n=4+4)
```

1 files changed, 3 insertions, 10 deletions

diff --git a/helpers/general.go b/helpers/general.go
index fa65265b8..a294455dd 100644
--- a/helpers/general.go
+++ b/helpers/general.go
@@ -27,12 +27,11 @@ import (
 	"unicode"
 	"unicode/utf8"
 
-	"github.com/cespare/xxhash/v2"
+	bp "github.com/gohugoio/hugo/bufferpool"
+
 	"github.com/spf13/afero"
 
 	"github.com/jdkato/prose/transform"
-
-	bp "github.com/gohugoio/hugo/bufferpool"
 )
 
 // FilePathSeparator as defined by os.Separator.
@@ -258,13 +257,7 @@ func SliceToLower(s []string) []string {
 	return l
 }
 
-// XxHashString takes a string and returns its xxHash hash.
-func XxHashString(f string) string {
-	h := xxhash.New()
-	h.WriteString(f)
-	hash := h.Sum(nil)
-	return hex.EncodeToString(hash)
-}
+// XXHashFromReader creates a xxHash hash from the given reader.
 
 // MD5String takes a string and returns its MD5 hash.
 func MD5String(f string) string {