doxygen: Remove lavu_internal group

There is no use in an internal group for a public API documentation.

libavutil/avutil.h

@@ -138,14 +138,6 @@
  *
  * @{
  *
- * @defgroup lavu_internal Internal
- *
- * Not exported functions, for internal usage only
- *
- * @{
- *
- * @}
- *
  * @defgroup preproc_misc Preprocessor String Macros
  *
  * @{

libavutil/intmath.h

@@ -33,11 +33,6 @@
 #   include "x86/intmath.h"
 #endif
 
-/**
- * @addtogroup lavu_internal
- * @{
- */
-
 #if HAVE_FAST_CLZ
 #if defined( __INTEL_COMPILER )
 #ifndef ff_log2
@@ -105,10 +100,6 @@ static av_always_inline av_const int ff_log2_16bit_c(unsigned int v)
 #define av_log2       ff_log2
 #define av_log2_16bit ff_log2_16bit
 
-/**
- * @}
- */
-
 /**
  * @addtogroup lavu_math
  * @{