V4L/DVB (8518): gspca: Remove the remaining frame decoding functions from the subdrivers.

SPCA505 and SPCA508 added in the pixel formats.
Decode functions and associated resources removed in spca505, 506 and 508.
The decode routines are now found in the V4L library.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>