Remove the q division by 3 for first frame.

This code/setting was actually not used (since speed features were not set on first frame,
until a recent change) and should be removed.

In CBR mode, the q value for the first frame can be controlled by setting
the target size via the parameters rc_buf_initial_sz (and max_intra_size_pct).

Change-Id: I65afc64972b36c449bd5a8c25800e65da5389066