doxygen: enable AUTOBRIEF, drop \brief

Especially for short descriptions, it is annoying to have to type \brief for
every single API doc.

Drop all \brief and enable the AUTOBRIEF feature of doxygen, which always takes
the first sentence of an API doc as the brief description.

Change-Id: I11a8a821b065a128108641a2a63fb5a2b1916e87
diff --git a/include/osmocom/gsm/a5.h b/include/osmocom/gsm/a5.h
index a2278f2..893fb02 100644
--- a/include/osmocom/gsm/a5.h
+++ b/include/osmocom/gsm/a5.h
@@ -32,10 +32,10 @@
  */
 
 /*! \file gsm/a5.h
- *  \brief Osmocom GSM A5 ciphering algorithm header
+ *  Osmocom GSM A5 ciphering algorithm header
  */
 
-/*! \brief Converts a frame number into the 22 bit number used in A5/x
+/*! Converts a frame number into the 22 bit number used in A5/x
  *  \param[in] fn The true framenumber
  *  \return 22 bit word
  */