Bluetooth: Remove unnecessary #ifdef for micro/nano main function

By calling the microkernel entry point 'main' there's no need to have
extra hacks to share the same c-file with both nano and micro
configurations.

Change-Id: I6623855a87ef8624b2bddb75b95079521e0eab8b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>