vht: Remove vht support

In the initial days of FRI, vht running in ami was the only supported
platform since it supported virtual socket interface. However, latest
ecosystem FVPs do support virtual socket interface. In addition, FRI now
supports FreeRTOS TCP/IP stack. Therefore, remove vht support.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>