Revert "avutil/opencl: is_compiled flag not being cleared in av_opencl_uninit"

Fixed build with opencl enabled
Found-by:  WJ Liu

This reverts commit 0f2359b869.

libavutil/opencl.c

@@ -611,9 +611,6 @@ void av_opencl_uninit(void)
         }
         opencl_ctx.context = NULL;
     }
-    for (i = 0; i < opencl_ctx.kernel_code_count; i++) {
-        opencl_ctx.kernel_code[i].is_compiled = 0;
-    }
     free_device_list(&opencl_ctx.device_list);
 end:
     if (opencl_ctx.init_count <= 0)