remove checking "optional" in schema validation

Although JSON Schema is not yet standardized, current pbnjson schema
validation was too ambiguous in checking required fields. (It had
"optional" and "requried".)
This patch removes "optional" schema validation.
Required fields should be described in "required" list as shown in
http://json-schema.org/examples.html

[GF-4065] Remove "optional" schema checking in pbnjson
Open-webOS-DCO-1.0-Signed-off-by: DongHeon Jang <dongheon.jang@lge.com>

Change-Id: I1a16ef43d1ca6aef504e6a6d266b26498c9434cf