st/mesa: fix assert to be simpler

I just noticed a warning with a non-debug build, but really
this could all be one line, and I'm not even 100% the assert
makes sense here.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_texture.c

@@ -508,8 +508,6 @@ static GLuint64
 st_create_texture_handle_from_unit(struct st_context *st,
                                    struct gl_program *prog, GLuint texUnit)
 {
-   struct gl_context *ctx = st->ctx;
-   struct gl_texture_object *texObj;
    struct pipe_context *pipe = st->pipe;
    struct pipe_sampler_view *view;
    struct pipe_sampler_state sampler;
@@ -519,8 +517,7 @@ st_create_texture_handle_from_unit(struct st_context *st,
 
    st_convert_sampler_from_unit(st, &sampler, texUnit);
 
-   texObj = ctx->Texture.Unit[texUnit]._Current;
-   assert(texObj);
+   assert(st->ctx->Texture.Unit[texUnit]._Current);
 
    return pipe->create_texture_handle(pipe, view, &sampler);
 }