Remove the xmesa_set_buffer() function. No longer needed since the

Remove the xmesa_set_buffer() function.  No longer needed since the
span functions pass in a gl_renderbuffer to indicate which color
buffer should be drawn into.  Optimized line/triangle routines are
smart enough to know which buffer to draw into as well.
The swrast->SetBuffer() routine should eventually be removed from
all drivers.