notmuch reply: Rename the mailing_list_munged_reply_to function

This function detects whether the address in the Reply-To header
already appears in either To or Cc. So give it a name that reflects
what it does (reply_to_header_is_redundant) rather than the old name
which described one possible use of the function, (as a simple
heuristic for detecting whether a mailing list had applied reply-to
munging).