Remove obsolete SchemaExtensions.txt

:Release Notes:

:Detailed Notes:
SchemaExtensions.txt described peculiarities of the former
implementation. Current implementation carefully follows the
revision 4 of the JSON schema specification.

:Testing Performed:
Trivial change.

:QA Notes:

:Issues Addressed:
[ANG-263] pnbjson: Remove SchemaExtensions.txt

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I4d8cb3f2cfa44f3ab48d6f44852f03b323f518e1
Reviewed-on: https://g2g.palm.com/4659
Reviewed-by: DCO Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>