Revert "fix MythTV C++ change, register is still valid C"

This reverts commit 1711295940e756102498dc020f8bff5f8e2c05a3.

The warnings are now suppressed in the files that include the internal headers.

mythtv/external/FFmpeg/libavcodec/get_bits.h

@@ -31,9 +31,7 @@
 
 /* MythTV: Prevent "ISO C++17 does not allow ‘register’ storage class
  * specifier" warning. */
-#ifdef __cplusplus
 #define register
-#endif
 
 #include "libavutil/common.h"
 #include "libavutil/intreadwrite.h"