gitk: Avoid handling the Return key twice in Add Branch

This reverts commit 63767d5.

A similar change was made as part of commit 76f1594, that added
bindings to all dialogs, and this duplication causes mkbrgo to be
called twice, the second time after the window has been destroyed.
As a result, an error window appears when the code tries to access
widgets.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -8270,7 +8270,6 @@ proc mkbranch {} {
     grid $top.id $top.sha1 -sticky w
     label $top.nlab -text [mc "Name:"]
     entry $top.name -width 40
-    bind $top.name <Key-Return> "[list mkbrgo $top]"
     grid $top.nlab $top.name -sticky w
     frame $top.buts
     button $top.buts.go -text [mc "Create"] -command [list mkbrgo $top]