tmux: drop deprecated status-utf8 option

tmux: drop deprecated status-utf8 option

From tmux's FAQ:

| Since the 1.0 release, tmux will turn on UTF-8 related options automatically
| (ie status-utf8, and utf8) if the above conditions are met.

In tmux's upstream there's also:

| commit 1b86f520ea1620628e569ea833c7b13306c18a4e
| Author: nicm <nicm>
| Date:   Thu Nov 12 11:09:11 2015 +0000
|
|     Nuke the utf8 and status-utf8 options and make tmux only a UTF-8
|     terminal. We still support non-UTF-8 terminals outside tmux, but inside
|     it is always UTF-8 (as when the utf8 and status-utf8 options were on).

Starting with tmux v2.2 this option is no longer supported.
While we still have tmux 1.9-6 in Debian/jessie there's at least
a backport of v2.3-4~bpo8+1 available in jessie-backports (and
corresponding v2.3-4 in testing/unstable), so the best we can do
is drop the configuration setting overall.

Thanks: Timo Boettcher for forwarding the bug report