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/coding/gsm0503_mapping.c b/src/coding/gsm0503_mapping.c
index 82c049c..15fd1b6 100644
--- a/src/coding/gsm0503_mapping.c
+++ b/src/coding/gsm0503_mapping.c
@@ -28,7 +28,7 @@
 /*! \addtogroup mapping
  *  @{
  *
- *  \brief GSM TS 05.03 burst mapping
+ *  GSM TS 05.03 burst mapping
  *
  *  This module contains burst mapping routines as specified in 3GPP TS
  *  05.03 / 45.003.