Bluetooth: Mesh: Remove unnecessary #ifdefs from beacon code

Many #ifdefs can be removed, but the UNPROV_BEACON_INT Kconfig
variable needs to be also made available also. This is done by making
its prompt (user selectability) optional rather than the option
itself. This approach is fine for "parameter style" options, but
should probably not be used for feature enabling options.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>