mesa/main: remove unused ClassID

This was probably useful in the past when we had DRI drivers, but it's
only ever written now. Let's get rid of it.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26418>

src/mesa/main/mtypes.h

@@ -2528,7 +2528,6 @@ struct gl_shared_state
  */
 struct gl_renderbuffer
 {
-   GLuint ClassID;        /**< Useful for drivers */
    GLuint Name;
    GLchar *Label;         /**< GL_KHR_debug */
    GLint RefCount;

src/mesa/main/renderbuffer.c

@@ -301,7 +301,6 @@ _mesa_init_renderbuffer(struct gl_renderbuffer *rb, GLuint name)
 {
    GET_CURRENT_CONTEXT(ctx);
 
-   rb->ClassID = 0;
    rb->Name = name;
    rb->RefCount = 1;
    rb->Delete = delete_renderbuffer;

src/mesa/state_tracker/st_manager.c

@@ -351,7 +351,6 @@ st_new_renderbuffer_fb(enum pipe_format format, unsigned samples, bool sw)
    }
 
    _mesa_init_renderbuffer(rb, 0);
-   rb->ClassID = 0x4242; /* just a unique value */
    rb->NumSamples = samples;
    rb->NumStorageSamples = samples;
    rb->Format = st_pipe_format_to_mesa_format(format);