cmd/cue/cmd: remove unused func mustParseFlags

All users of mustParseFlags were removed in commit 0489caa3a13c
("cmd/cue/cmd: use testscript for top level command testing")

Closes #373
https://github.com/cuelang/cue/pull/373

GitOrigin-RevId: 5dfd119398a7f18f00d3130d9c0ed2da3131e805
Change-Id: Iba25ed2270f0c3018e575b41188075704ee8dbaf
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5901
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cmd/cue/cmd/common.go b/cmd/cue/cmd/common.go
index a61fe08..ddeec68 100644
--- a/cmd/cue/cmd/common.go
+++ b/cmd/cue/cmd/common.go
@@ -21,9 +21,7 @@
 	"os"
 	"path/filepath"
 	"strings"
-	"testing"
 
-	"github.com/spf13/cobra"
 	"golang.org/x/text/language"
 	"golang.org/x/text/message"
 
@@ -60,12 +58,6 @@
 
 var inTest = false
 
-func mustParseFlags(t *testing.T, cmd *cobra.Command, flags ...string) {
-	if err := cmd.ParseFlags(flags); err != nil {
-		t.Fatal(err)
-	}
-}
-
 func exitIfErr(cmd *Command, inst *cue.Instance, err error, fatal bool) {
 	exitOnErr(cmd, err, fatal)
 }