self update: remove not possible exit codes

cmd/restic/cmd_self_update.go

@@ -38,9 +38,6 @@ EXIT STATUS
 
 Exit status is 0 if the command was successful.
 Exit status is 1 if there was any error.
-Exit status is 10 if the repository does not exist.
-Exit status is 11 if the repository is already locked.
-Exit status is 12 if the password is incorrect.
 `,
 		DisableAutoGenTag: true,
 		RunE: func(cmd *cobra.Command, args []string) error {