gitk: Fix bug in accessing undefined "notflag" variable

As pointed out by Johannes Sixt and Alexander Gavrilov, commit
2958228 ("gitk: Fix switch statement
in parseviewargs") exposed a latent bug in that $notflag was never
initialized.  Since it isn't used either, this removes it entirely.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -199,11 +199,7 @@ proc parseviewargs {n arglist} {
 		set nextisval 1
 		lappend glflags $arg
 	    }
-	    "--not" {
-		set notflag [expr {!$notflag}]
-		lappend revargs $arg
-	    }
-	    "--all" {
+	    "--not" - "--all" {
 		lappend revargs $arg
 	    }
 	    "--merge" {