webp-container-spec: remove redundant sentence
in the note regarding metadata the possibility of multiple XMP/EXIF
chunks was already covered in the previous sentences.
based on comments from:
@@ -686,8 +686,7 @@ If this chunk is not present, sRGB SHOULD be assumed.
Metadata can be stored in 'EXIF' or 'XMP ' chunks.
There SHOULD be at most one chunk of each type ('EXIF' and 'XMP '). If there
-are more such chunks, readers MAY ignore all except the first one. Also, a file
-may possibly contain both 'EXIF' and 'XMP ' chunks.
+are more such chunks, readers MAY ignore all except the first one.
The chunks are defined as follows:
