Remove tw_policy_decision.*

Remove tw_policy_decision.*

There were two distinct classes: Ewk_Policy_Decision and
_Ewk_Policy_Decision. _Ewk_Policy_Decision was a subclass of
tizen_webview::PolicyDecision. Ewk_Policy_Decision owned an
_Ewk_Policy_Decision and forwarded it it's method.

This patch simplifies everything: there is now only
_Ewk_Policy_Decision, and Ewk_Policy_Decision is an alias to it.

Change-Id: I74c5187dda6afc7f3cd362d0107d1f8e52ec1245
Signed-off-by: Arnaud Renevier <a.renevier@samsung.com>