Revert "Restore cache_all flag removal"

This reverts commit 713d906.

We should deprecate it rather the remove it abruptly.

bundler/lib/bundler/cli.rb

@@ -440,10 +440,9 @@ def outdated(*gems)
     end
 
     desc "cache [OPTIONS]", "Locks and then caches all of the gems into vendor/cache"
-    unless Bundler.feature_flag.cache_all?
-      method_option "all",  :type => :boolean,
-                            :banner => "Include all sources (including path and git)."
-    end
+    method_option "all",  :type => :boolean,
+                          :default => Bundler.feature_flag.cache_all?,
+                          :banner => "Include all sources (including path and git)."
     method_option "all-platforms", :type => :boolean, :banner => "Include gems for all platforms present in the lockfile, not only the current one"
     method_option "cache-path", :type => :string, :banner =>
       "Specify a different cache path than the default (vendor/cache)."