Correctly clean up lsp--update-signature-help-hook

[sstraust]

lsp--update-signature-help-hook is not correctly removed when lsp-managed-mode is disabled.

Steps to reproduce:
open https://git.sr.ht/~davidcode/emacs-lsp-problem, and then try to disable lsp-mode and lsp-managed-mode

check the value of the variable post-self-insert-hook.

verified this change by running lsp-managed-mode (to turn it on and off) with the applied changes, and the post hook is no longer visible.

This has a real impact, as it pops up documentation strings, even after LSP mode is turned off.

Paired with David Howard on finding this issue.

[sstraust]

I see that my errors mention that lsp-byte-compilation test failed. I'm assuming there's some compilation step I'm supposed to run before submitting this change, but I'm not sure what it is.

Any pointers on where to find it?

(i did read the contributing guidelines, but this is my first attempt to contribute to a major elisp project, so i might be very dumb lol)