swrast: Remove redundant test of the visual's accumulation buffer size

swrast: Remove redundant test of the visual's accumulation buffer size

If the visual doesn't have an accumulation buffer, the renderbuffer
passed into _swrast_clear_accum_buffer will be NULL anyway.  There is
no reason the check the visual.  Moreover, the test erroneously checks
the context's visual instead of the visual of the current DrawBuffer.
With FBOs these may be different.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>