Bluetooth: controller: Remove timestamp from HCI

In order to avoid the build system complications that come from
including a timestamp, remove it by default from the version string in
HCI Vendor Extensions. Users can still include a unique identifier, be
it timestamp or not, using the CONFIG_BT_CTLR_HCI_VS_BUILD_INFO Kconfig
option.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>