Revert "BUG-890: Update ietf-inet-types and ietf-yang-types with RFC6991"

This reverts commit a4ef786b6a791872b00aede1a7fa862d2ea0bd55.

Change-Id: Ibb5438f67de40bf39e8e5b72bb8465640bf35b8e
Signed-off-by: Robert Varga <rovarga@cisco.com>