perf tui: Make <- exit menus too

In fact it is now added to the hot key list when newt_form__new is used,
allowing us to remove the explicit assignment in all its users.

The visible change is that <- will exit the menu that pops up when -> is
pressed (and Enter when callchains are not being used).

Suggested-by: Ingo Molnar <mingo@elte.hu>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>