Remove 6GHz capability query through HAL

It was decided to perform the check on the device support of 6GHz
through overlay configs instead of through HAL.
This commit removes the Hidl API changes that were used for that
check.

Bug: 145936758
Bug: 139354972
Test: Manual
Test: VTS test
Test: Unit test: hardware/interfaces/wifi/1.4/default/tests/runtests.sh
Change-Id: I81fdf603a668a71e3e875211367111fffee20e54