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/core/panic.h b/include/osmocom/core/panic.h
index 5d575c4..92c557f 100644
--- a/include/osmocom/core/panic.h
+++ b/include/osmocom/core/panic.h
@@ -8,7 +8,7 @@
 
 #include <stdarg.h>
 
-/*! \brief panic handler callback function type */
+/*! panic handler callback function type */
 typedef void (*osmo_panic_handler_t)(const char *fmt, va_list args);
 
 extern void osmo_panic(const char *fmt, ...);