gdb/mi: remove warning about mi1

Remove a warning about mi1.  mi1 was removed in 975249ff4e26 ("Remove MI
version 1").  It is no longer possible to reach this warning, since
trying to use interpreter mi1 bails out before:

    $ ./gdb -nx -q --data-directory=data-directory -i mi1
    Interpreter `mi1' unrecognized

Change-Id: Ie43b21e01bca1407995150c729531a70ee662003
Approved-By: Tom Tromey <tom@tromey.com>