cnss2: Remove deprecated CNSS_BDF_DUMMY type

CNSS_BDF_DUMMY type board data file is no longer used. Hence remove
it. A BDF file with regular name and dummy content should be sent as
dummy BDF instead of just sending a dummy string.

Change-Id: I7483a6801584f56ac4e87e2074280291c9734472
Signed-off-by: Yue Ma <yuem@codeaurora.org>