batman-adv: Drop unused headers in trace.h

The commit 9abc2918 ("batman-adv: tracing: Use the new __vstring()
helper") removed the usage of WARN_ON_ONCE and __dynamic_array in this
file. But it was forgotten to adjust the headers accordingly (dropping the
now no longer used ones).

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>