Remove unused JL_HAVE_SIGALTSTACK code

[fingolfin]

Nothing is ever activating this, and it probably hasn't been tested in a while.

As far as I can tell, this code path was added by @vtjnash in 082d7d2

I have no particular beef with this code other than "it complicates the thread / task logic, so dealing with this requires understanding one more moving part, removing it makes it simpler". If it is decided that this code should rather be kept, "just in case" (which, though), that's equally fine by me. Thought perhaps a comment could be added explaining the motivation for keeping it even thought it is currently always disabled?

[fingolfin]

This needs a rebase and probably another one once PR #54569 is merged; I am happy to do those but I'd rather wait until I hear an "OK this is sensible" -- if this PR is rejected there is no point in updating it :-)

[fingolfin]

@vtjnash any thoughts on this PR? Pinging you since it seems you added the JL_HAVE_SIGALTSTACK code in the first place, so may know reasons why it should be kept, or perhaps even situations were it is used after all?

My main motivation for removing it was to lower complexity when trying to understand how everything in the task system fits together, and what code is used when.

[vtjnash]

I don't have any insight into what options people use on esoteric configurations

[gbaraldi]

I would be fine with removing it and if someone complains we can revert?

[fingolfin]

@gbaraldi that's just my thought!

Disclaimer: I actually plan a follow-up PR which would slightly complicate the revert (by modifying some of the same lines as this PR), but not in a fundamental way. And I'd be happy to either assist with or even perform the revert if needed.