Revert "vhost: Reintroduce kthread API and add mode selection"

This reverts commit b2a3018e which is
commit 7d9896e9 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I5c30129aff6637f818039f376468aa1298c0f1db
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>