TODO: remove CURLOPT_DNS_USE_GLOBAL_CACHE entry

Commit 7c5837e7

 deprecated the option
making it a manual code-edit operation to turn it back on. The removal
process has thus started and is now documented in docs/DEPRECATE.md so
remove from the TODO to avoid anyone looking for something to pick up
spend cycles on an already in-progress entry.

Reviewed-by: Daniel Stenberg <daniel@haxx.se>