orinoco: remove unused array encaps_hdr and macro ENCAPS_OVERHEAD

Array encaps_hdr and macro ENCAPS_OVERHEAD are declared but are
not being used, hence they are redundant and can be removed.

Cleans up clang warning:
warning: 'encaps_hdr' defined but not used [-Wunused-const-variable=]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>