vbo: remove bogus assert

grow_vertex_storage may call wrap_filled_vertex, which will
trigger the assert incorrectly because the new size will be
smaller than 'new_size' but it's correct because
'vertex_store->used' has been reset to 0.

Fixes: a08baaff976 ("vbo/dlist: fix indentation in vbo_save_api.c")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20748>
(cherry picked from commit 491f6b138ecbe938471f7ee1bc339b32c61eed07)