git-gui: remove Tcl 8.4 workaround on 2>@1 redirection

git-gui: remove Tcl 8.4 workaround on 2>@1 redirection

Since b792230 ("git-gui: Show a progress meter for checking out files",
2007-07-08), git-gui includes a workaround for Tcl that does not support
using 2>@1 to redirect stderr to stdout. Tcl added such support in
8.4.7, released in 2004, and this is fully supported in all 8.5
releases.

As git-gui has a hard-coded requirement for Tcl >= 8.5, the workaround
is no longer needed. Delete it.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>