[fxicfg] Remove diff flag.

We'll use git to diff configs on CQ in repositories that
contain starlark configs. We don't need this flag.

Bug: IN-1102 #comment
Change-Id: I7a2b73ce23b370744df55a047e09054ebb8a6cde
diff --git a/cmd/fxicfg/generate.go b/cmd/fxicfg/generate.go
index 4c35674..37ba30b 100644
--- a/cmd/fxicfg/generate.go
+++ b/cmd/fxicfg/generate.go
@@ -16,11 +16,7 @@
 	luci "go.chromium.org/luci/starlark/interpreter"
 )
 
-// TODO(IN-1102): Delete all files in the output dir that weren't generated by this
-//   run to prevent "zombie" configs.
-type GenerateCommand struct {
-	diffOnly bool
-}
+type GenerateCommand struct{}
 
 func (*GenerateCommand) Name() string {
 	return "generate"
@@ -34,9 +30,7 @@
 	return "generates infrastructure configs from Starlark sources"
 }
 
-func (cmd *GenerateCommand) SetFlags(f *flag.FlagSet) {
-	flag.BoolVar(&cmd.diffOnly, "diff", false, "Whether to display changes and exit without writing files")
-}
+func (cmd *GenerateCommand) SetFlags(f *flag.FlagSet) {}
 
 func (cmd *GenerateCommand) Execute(ctx context.Context, f *flag.FlagSet, _ ...interface{}) subcommands.ExitStatus {
 	if err := cmd.validateFlags(f); err != nil {
@@ -57,11 +51,6 @@
 		return err
 	}
 
-	if cmd.diffOnly {
-		// TODO(IN-1102)
-		return errors.New("unimplemented: -diff")
-	}
-
 	if err := fxicfg.CommitState(state); err != nil {
 		return err
 	}