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/gsm0341.c b/src/gsm/gsm0341.c
index 2592b54..0e3e453 100644
--- a/src/gsm/gsm0341.c
+++ b/src/gsm/gsm0341.c
@@ -31,7 +31,7 @@
  *  @{
  */
 
-/*! \brief Encode a 3GPP TS 03.41 SMS-CB message
+/*! Encode a 3GPP TS 03.41 SMS-CB message
  *  \param[in] ctx talloc allocation context
  *  \param[in] geo_scope Geographic Scope
  *  \param[in] msg_code Message Code