avcodec/libx264: remove FF_CODEC_CAP_INIT_THREADSAFE flag

Libx264 uses strtok which is not thread safe. Strtok is used in
x264_param_default_preset in param_apply_tune in x264/common/base.c.
Therefore the flag must be removed.

x264 fixed the issue, once the fix is pushed to stable, an #if can be added
to re-enable the flag based on X264_BUILD number.

Fixes ticket #7446.

Signed-off-by: Marton Balint <cus@passwd.hu>