RFR: Remove _keepalive WeakHandle remnants.

[Coleen Phillimore]

On Mon, 10 May 2021 12:37:25 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> Since I went down the path of looking at _keepalive and found that the code for it was removed, I think the rest of it should be also.
> Tested with loom-tier1, 2.

@fisk did I find it all?

-------------

PR: https://git.openjdk.java.net/loom/pull/41