Revert "Show allowed channel types in ISUPPORT(005) numeric only"

Revert "Show allowed channel types in ISUPPORT(005) numeric only"

The ISUPPORT(005) numeric lists only channel prefixes which are listed
in the "AllowedChannelTypes" configuration option. And if this is the
empty string ("") for example, this now results in IRC clients assuming
"oh, no channel prefix characters at all, so no channels at all, so no
PRIVMSG can go to any channel" -- which is not the case when there are
pre-defined channel set up or other servers still having channels!

So "allowed channel types" != "supported channel types", and we always
have to list all supported ones in the ISUPPORT(005) numeric!

This reverts commit 4b7e8db418340576c95f1edad8470b66d6fe886d.

Closes #285.