pkg-generic: remove the .stamp_rsync_sourced fake stamp file

The only reason for the .stamp_rsync_sourced fake stamp file target to
exist was to handle the SOURCE_CHECK operation on packages using the
OVERRIDE_SRCDIR mechanism. Now that this is handled directly inside
$(1)-source-check, there is no longer any need for this part of the
code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>