Remove unnecessary WKWebView/UIWebview logic

[josh-m-sharpe]

Following up from our convo here:
Telerik-Verified-Plugins#185

This PR merges in the changes I was making in that PR, which pretty much does the same thing, but also removes some additional unnecessary logic.

[tlacroix]

Hi @josh-m-sharpe, thanks for following up with your PR on the original project. I looked at it just after I submitted mine (on Telerik's repo), and it seemed a lot cleaner. I'll just need a bit of time to test it with a few of my apps before I merge it since there's no automated testing, but just by looking at the code, I'm 99.999% sure everything's fine. Thanks again!

[tlacroix]

Tested and all good. Just one change request, since your PR removes all support for UIWebView:

Please add, in plugin.xml:

<plugin ...>
    <engines>
        ...
        <engine name="cordova-ios" version=">=5.1.1"/>
        ...
    </engine>
    ...
</plugin>

[josh-m-sharpe]

@tlacroix would you mind adding/testing that bit? I don't use cordova, so I have no means to test/confirm that part.