Remove imagemagick strip (alshedivat#2028)

I had added -strip arg to the imagemagick call as it is generally
desirable to strip metadata from images before publishing them, however
it seems to occasionally cause issues with jekyll-imagemagick
auto-regeneration, regenerating images even when they have not changed.

_config.yml

@@ -345,7 +345,7 @@ imagemagick:
     - ".tiff"
     - ".gif"
   output_formats:
-    webp: "-quality 85 -strip"
+    webp: "-quality 85"
 
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