gallium: Remove temporary hack for the absence of a discard flag.

PIPE_BUFFER_USAGE_DISCARD flag now exists.
diff --git a/src/gallium/include/pipe/p_inlines.h b/src/gallium/include/pipe/p_inlines.h
index 5fbd62a..6949f56 100644
--- a/src/gallium/include/pipe/p_inlines.h
+++ b/src/gallium/include/pipe/p_inlines.h
@@ -63,13 +63,6 @@
    if(screen->buffer_map_range) {
       unsigned offset = 0;
       unsigned length = buf->size;
-
-      /* XXX: Actually we should be using/detecting DISCARD
-       * instead of assuming that WRITE implies discard */
-      if((usage & PIPE_BUFFER_USAGE_CPU_WRITE) &&
-         !(usage & PIPE_BUFFER_USAGE_DISCARD))
-         usage |= PIPE_BUFFER_USAGE_CPU_READ;
-
       return screen->buffer_map_range(screen, buf, offset, length, usage);
    }
    else