git-gui: translation binary defines iso8859-1

git-gui: translation binary defines iso8859-1

git-gui has many cases where -translation binary and -encoding binary
are configured on the same channel. But, -translation binary defines a
binary channel, which sets up -encoding iso8859-1 as part of its work.
Tcl 8.x defines -encoding binary as an alias of -encoding iso8859-1, and
this alias is deleted in Tcl 9.0.  Let's delete the redundant encoding
definition now.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>