Update draft/rename implementation

Link to the new draft PR:
<https://github.com/ircv3/ircv3-specifications/pull/420>

Changes in the spec:

- Use standard replies instead of numerics:
  <https://github.com/ircv3/ircv3-specifications/pull/420/files#diff-70e90beef48dc9cf5d784d1e179ea822R44>
- Allow RENAME to a different case:
  <https://github.com/ircv3/ircv3-specifications/pull/420/files#diff-70e90beef48dc9cf5d784d1e179ea822R42>

This commit makes oragono send the PART-JOIN fallback even on case-only
changes. This is so that clients don't have to worry about oragono's
UTF8 casefolding. See the following comments for further info:
<https://github.com/ircv3/ircv3-specifications/pull/420#issuecomment-668770837>

Misc fixes:

- Remove unused variable,
- Add missing calls to utils.SafeErrorParam,
- Don't fill replies with the user-provided "oldName", for the same
  reason as sending the PART-JOIN fallback.

irc/numerics.go

@@ -168,8 +168,6 @@ const (
 	ERR_CANNOTSENDRP              = "573"
 	RPL_WHOISSECURE               = "671"
 	RPL_YOURLANGUAGESARE          = "687"
-	ERR_CHANNAMEINUSE             = "692"
-	ERR_CANNOTRENAME              = "693"
 	ERR_INVALIDMODEPARAM          = "696"
 	ERR_LISTMODEALREADYSET        = "697"
 	ERR_LISTMODENOTSET            = "698"