Fastradio was a trick where the ClientHello was padding to at least 1024
bytes in order to trick some mobile radios into entering high-power mode
immediately. After experimentation, the feature is being dropped.

This change also tidies up a bit of the extensions code now that
everything is using the new system.

Change-Id: Icf7892e0ac1fbe5d66a5d7b405ec455c6850a41c
Reviewed-on: https://boringssl-review.googlesource.com/5466
Reviewed-by: Adam Langley <agl@google.com>