mesa: remove gl_renderbufer::PutMonoRow() and PutMonoValues()

The former was only used for clearing buffers.  The later wasn't used
anywhere!  Remove them and all implementations of those functions.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/main/renderbuffer.c

@@ -79,9 +79,7 @@ _mesa_init_renderbuffer(struct gl_renderbuffer *rb, GLuint name)
    rb->GetValues = NULL;
    rb->PutRow = NULL;
    rb->PutRowRGB = NULL;
-   rb->PutMonoRow = NULL;
    rb->PutValues = NULL;
-   rb->PutMonoValues = NULL;
 }