Revert "cfg80211: Add new wiphy flag WIPHY_FLAG_DFS_OFFLOAD"

This reverts commit b07c35c1.
Upstream kernel has defined the flag NL80211_EXT_FEATURE_DFS_OFFLOAD
for the same purpose, therefore remove WIPHY_FLAG_DFS_OFFLOAD from
cfg80211.

Change-Id: I729e84094a335595ee4cd86f721246dfc307ae61
CRs-fixed: 2265562
Signed-off-by: Peng Xu <pxu@codeaurora.org>