gems: drop Mechanize, HTTParty, and Sinatra gems.

app/logical/danbooru/http.rb

@@ -22,8 +22,8 @@ module Danbooru
       @http ||=
         ::Danbooru::Http::ApplicationClient.new
         .timeout(DEFAULT_TIMEOUT)
-        .headers(Danbooru.config.http_headers)
         .headers("Accept-Encoding" => "gzip")
+        .headers("User-Agent": "#{Danbooru.config.canonical_app_name}/#{Rails.application.config.x.git_hash}")
         .use(:auto_inflate)
         .use(:retriable)
         .use(redirector: { max_redirects: MAX_REDIRECTS })

app/logical/pixiv_api_client.rb

@@ -155,7 +155,6 @@ class PixivApiClient
 
   def api_client
     http.headers(
-      **Danbooru.config.http_headers,
       "Referer": "http://www.pixiv.net",
       "Content-Type": "application/x-www-form-urlencoded",
       "Authorization": "Bearer #{access_token}"

app/logical/sources/strategies/base.rb

@@ -141,7 +141,7 @@ module Sources
       # Subclasses should merge in any required headers needed to access resources
       # on the site.
       def headers
-        Danbooru.config.http_headers
+        {}
       end
 
       # Returns the size of the image resource without actually downloading the file.