Skip erc-ignored-user-p when erc-ignore-list is empty

Skip erc-ignored-user-p when erc-ignore-list is empty

* lisp/erc/erc-backend.el (erc-server-PRIVMSG): Don't bother running
`erc-ignored-user-p' and `erc-ignored-reply-p' when their associated
options are null.  The option `erc-ignore-list' is buffer-local when
set, and `erc-ignored-user-p' looks for it in the server buffer.
Moreover, all functions that set it, like `erc-cmd-IGNORE' and
`erc-cmd-UNIGNORE', do so in the server buffer.  And the response
handler in question only runs in server buffers, so this shouldn't
break anything.  Also, remove stray call to reassign trailing response
contents.
* lisp/erc/erc-common.el (erc-get-server-user): Rearrange so
`erc-with-server-buffer' doesn't have to switch to the server buffer
because `erc-downcase' can run in channels as well.
* lisp/erc/erc.el (erc-ignored-user-p): Add comment.  (Bug#67677)