Fix issue 6064: Inline images don't display on KitKat (v. 2)

(New pull request in lieu of #441)

Eliminate the invocation of
WebSettings.setBlockNetworkImage(boolean flag),
thus maintaining the the default setting of "false".

On Android versions prior to KitKat, this setting has no
effect on inline image attachments loaded with content:
URIs.  Such images would load regardless.

With KitKat, this setting does have an effect -- a
setting of "true" will block image attachments loaded
with content: URIs.

By removing this call, K-9 Mail behaves the same on KitKat
as on earlier Android versions, and the behavior on earlier
versions is unchanged.