"slice mode" option seems to be unnecessary since it could be
determined by -slices/max_nal_size.

default:        SM_FIXEDSLCNUM_SLICE mode with cpu-number slices.
-slices N:      SM_FIXEDSLCNUM_SLICE mode with N slices.
-max_nal_size:  SM_SIZELIMITED_SLICE mode with limited size slices.

Add FF_API_OPENH264_SLICE_MODE macro to remove this option after
LIBAVCODEC_VERSION_MAJOR = 59.
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Martin Storsjö <martin@martin.st>