Revert "cpu/avr/radio/rf230bb/rf230bb.c: pulse SLP_TR after writing the frame"

This reverts commit d8436e538af90fdd85b6c24a5cce2954c5fdca8a.

The original sequence is intentional and make sense.
See https://github.com/contiki-os/contiki/pull/295
cpu/avr/radio/rf230bb/rf230bb.c (rf230_transmit): extend comment on tx sequence

Explain that the transmission is intentionally started before copying the
frame to the buffer.