Simplify check before git install

No need to check for both variables, because they're always set together.
Also, this harmonizes with the CURL equivalent just above.

Library/Homebrew/cmd/update.sh

@@ -384,9 +384,8 @@ EOS
   fi
 
   if ! git --version &>/dev/null ||
-     [[ ( -n "$HOMEBREW_SYSTEM_GIT_TOO_OLD" ||
-          -n "$HOMEBREW_FORCE_BREWED_GIT" ) &&
-        ! -x "$HOMEBREW_PREFIX/opt/git/bin/git" ]]
+     [[ -n "$HOMEBREW_FORCE_BREWED_GIT" &&
+      ! -x "$HOMEBREW_PREFIX/opt/git/bin/git" ]]
   then
     # we cannot install brewed git if homebrew/core is unavailable.
     [[ -d "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core" ]] && brew install git