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/src/gsm/gsm0411_smc.c b/src/gsm/gsm0411_smc.c
index 03016b9..dab83eb 100644
--- a/src/gsm/gsm0411_smc.c
+++ b/src/gsm/gsm0411_smc.c
@@ -62,7 +62,7 @@
 
 /*! \addtogroup sms
  *  @{
- *  \brief Point-to-Point (PP) Short Message Service (SMS) as per TS 04.11
+ *  Point-to-Point (PP) Short Message Service (SMS) as per TS 04.11
  */
 
 static void cp_timer_expired(void *data);