Remove the cache validation

The cache validation worked based on timestamps. I believe this
may be unreliable in a workflow pipeline with cached assets.

Instead, we now rely on the digest of the file. If the source file
changes then so should the digest. And therefore it would not find
a file in the cache.

Author: Garanas

lib/jekyll/transcode-image-filters.rb

@@ -52,14 +52,6 @@ def _in_cache? (absolute_path_source, absolute_path_destination)
       File.exist?(absolute_path_destination)
     end
 
-    # Determine whether the file in the cache is still valid. It does so by comparing the modification times of the files.
-    # @param absolute_path_source [String] As an example: "C:/jekyll/jekyll-transcode-image-filters"
-    # @param absolute_path_destination [String] As an example: "/assets/image-a.png" or "/assets/image-a.bmp"
-    # @return [Boolean] If true, the file should be skipped.
-    def _valid_cache? (absolute_path_source, absolute_path_destination)
-      File.mtime(absolute_path_destination) >= File.mtime(absolute_path_source)
-    end
-
     # Read, process, and write the (new) image to disk.
     # @param absolute_path_source [String] As an example: "C:/jekyll/jekyll-transcode-image-filters/assets/image-a.png"
     # @param absolute_path_destination [String] As an example: "C:/jekyll/jekyll-transcode-image-filters/assets/image-a-processed.png" 
@@ -96,7 +88,7 @@ def _transcode_image (relative_source_path, cache_dir, resolution, format)
       # Guarantee the existence of the cache directory
       FileUtils.mkdir_p(absolute_path_cache)
 
-      if _in_cache?(absolute_path_source, absolute_path_destination) && _valid_cache?(absolute_path_source, absolute_path_destination)
+      if _in_cache?(absolute_path_source, absolute_path_destination)
         # if the file is cached and valid we can just return the relative path
         return relative_path_destination
       else