Wifi: Remove capability for 6GHz

It was decided to base device capability for support of 6GHz band
on overlay configs rather than HAL capability bitmask.
Hence, this commit removes the 6GHz feature flag.

Bug: 145936758
Bug: 139354972
Test: Manual
Change-Id: I0c72161a31452bd0fab226f9f92f0aa859d3f5e4
1 file changed