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/codec/codec.h b/include/osmocom/codec/codec.h
index 05da911..3e9b688 100644
--- a/include/osmocom/codec/codec.h
+++ b/include/osmocom/codec/codec.h
@@ -42,7 +42,7 @@
        AMR_GOOD = 1
 };
 
-/*! \brief Check if given AMR Frame Type is a speech frame
+/*! Check if given AMR Frame Type is a speech frame
  *  \param[in] ft AMR Frame Type
  *  \returns true if AMR with given Frame Type contains voice, false otherwise
  */