libavutil: drop offsetof() fallback definition

The only compiler I have that does not define the standard
offsetof() macro is "Bruce's C Compiler", a simple compiler
for producing 8/16-bit 8086 code, usually for use in early
stages of PC booting.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/internal.h

@@ -56,10 +56,6 @@ struct AVDictionary {
 #    define INT_BIT (CHAR_BIT * sizeof(int))
 #endif
 
-#ifndef offsetof
-#    define offsetof(T, F) ((unsigned int)((char *)&((T *)0)->F))
-#endif
-
 /* debug stuff */
 
 #define av_abort()      do { av_log(NULL, AV_LOG_ERROR, "Abort at %s:%d\n", __FILE__, __LINE__); abort(); } while (0)