brcmfmac: drop unused pm_block vif attribute

This attribute was added 3 years ago by
commit 3eacf866 ("brcmfmac: introduce brcmf_cfg80211_vif structure")
but it remains unused since then. It seems we can safely drop it.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>