Stop forcing Ext.USE_NATIVE_JSON as false (use Ext JS default)

[fabriciomurta]

Ext JS 6.6.0 has changed the setting to true, but it is incompatible with several parts of the code that takes advantage of the eval() nature of Ext JS's JSON implementation.

As it introduces a breaking change, and also will mean structural changes on Ext.NET direct method/events code, it should only follow to a major version release, so Ext.NET was set up to force using Ext JS implementation by default by #1604.

This change can only make it out to be a default in the v5 release of Ext.NET due to the breaking change nature.

The change here would be no longer force Ext.USE_NATIVE_JSON = false and use whatever Ext JS is set up to by default.

[fabriciomurta]

With this, former calls from Ext.NET to Ext.decode() that were meant to be evaluated as javascript was changed to actual eval() calls, based on what Ext.decode() does when USE_NATIVE_JSON=false.