overlays: Deprecate and delete the sdtweak overlay

The sdtweak overlay has been superseded by the board-specific
sd_* parameters such as sd_poll_once, sd_overclock etc.

For example, replace:

    dtoverlay=sdtweak,poll_once

with:

    dtparam=sd_poll_once

Signed-off-by: Phil Elwell <phil@raspberrypi.com>