regression fix due to `Add a UCI config option to enable or disable local extensions`

regression due to commit:3b827b9a

The following msg could be seen during asterisk startup.

sed: /tmp/tmp.GcepcM/extensions/provider: No such file or directory
sed: /tmp/tmp.GcepcM/extensions/provider: No such file or directory

and the |LOCAL_EXTENSIONS| which should be replaced or deleted were still there due to the above issue.

;
; Contexts for SIP Provider
;
[sip0]
; Main context, handles all normal calls for this Provider
|LOCAL_EXTENSIONS|

This lead all local extensions stop working if a sip provider is registered.