Don't set GIT_DIR when staging git clones

GIT_DIR is convenient, but it does not work when both a formula and
subformula try to stage a git checkout, as git will use the main
formula's repository when staging the subformula.

Stop setting it, and restore the kludge to the git-multipush formula.

Fixes Homebrew/homebrew#10893.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/download_strategy.rb

@@ -393,7 +393,6 @@ class GitDownloadStrategy < AbstractDownloadStrategy
         safe_system 'git', 'submodule', '--quiet', 'foreach', '--recursive', sub_cmd
       end
     end
-    ENV['GIT_DIR'] = cached_location+'.git'
   end
 end