Strategy: Temporarily remove response caching

The simple approach here caches all header or body content from
responses, so memory usage continually grows with each fetch. This
becomes more of a notable issue with long livecheck runs (e.g.,
`--tap homebrew/core`).

Instead, we should only cache the header/body for URLs that we know
will be fetched more than once in a given run. Being able to
determine which URLs will be fetched more than once requires
structural changes within livecheck strategies, so this will take a
bit of work to implement.

I've been working on this off and on and I'll introduce a more
sophisticated method of livecheck-wide caching in a later PR. In the
interim time, it's best to remove this caching behavior until I've
finished working on an approach that provides benefits (reducing
duplicate fetches) while minimizing detriments (increased memory
usage).

Library/Homebrew/livecheck/strategy.rb

@@ -82,10 +82,6 @@ module Homebrew
       end
 
       def self.page_headers(url)
-        @headers ||= {}
-
-        return @headers[url] if @headers.key?(url)
-
         headers = []
 
         [:default, :browser].each do |user_agent|
@@ -113,15 +109,14 @@ module Homebrew
                         .to_h.transform_keys(&:downcase)
           end
 
-          return (@headers[url] = headers) if status.success?
+          return headers if status.success?
         end
 
         headers
       end
 
       def self.page_content(url)
-        @page_content ||= {}
-        @page_content[url] ||= URI.parse(url).open.read
+        URI.parse(url).open.read
       end
     end
   end