bin/main: Remove duplicated usage output on unknown commands
bin/main: Remove duplicated usage output on unknown commands

It's been this way for a long time, though not forever; I went
back to v2014.11 as a random choice and it worked then.  Not
going to bother doing a full archive search for this though.
Anyone who wants more context can help themselves.

Closes: https://github.com/ostreedev/ostree/issues/1096

Closes: #1106
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree