avfilter/buffersrc: remove redundant flag

avfilter/buffersrc: remove redundant flag

!(c->pix_fmt != AV_PIX_FMT_NONE || c->got_format_from_params)

equals

(c->pix_fmt == AV_PIX_FMT_NONE) && !c->got_format_from_params

1. When (c->pix_fmt == AV_PIX_FMT_NONE) is true, got_format_from_params is
always false, the flag doesn't contribute to the result.

2. When the first part is false, the second part doesn't matter, the flag
doesn't contribute to the result.

The result only depends on c->pix_fmt.