~/.muttrc: Remove {1,2,3} folder change keys.

Those are handled by the sidebar and quicknav.

.muttrc

macro index = <tag-prefix>
macro index * <tag-entry>
## changing mailboxes
macro index 1 "<change-folder>=lu/INBOX<enter>"
macro index 2 "<change-folder>=htwk.imn/INBOX<enter>"
macro index 3 "<change-folder>=htwk/INBOX<enter>"
bind  index n next-unread-mailbox
# colors