Remove unnecessary parameter from ff_thread_init() and fix behavior

thread_count passed to ff_thread_init() is only used to set AVCodecContext.
thread_count, and can be removed. Instead move it to the legacy implementation
of avcodec_thread_init().

This also fixes the problem that calling avcodec_thread_init() with pthreads
enabled did not set it since ff1efc52.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>