Remove extern C blocks from lib/* internal APIs (except xxhash.h)

lib/common/zstd_trace.h

@@ -13,10 +13,6 @@
 
 #include <stddef.h>
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
 /* weak symbol support
  * For now, enable conservatively:
  * - Only GNUC
@@ -157,8 +153,4 @@ ZSTD_WEAK_ATTR void ZSTD_trace_decompress_end(
 
 #endif /* ZSTD_TRACE */
 
-#if defined (__cplusplus)
-}
-#endif
-
 #endif /* ZSTD_TRACE_H */