These links were actually defined backwards: the highlight groups
actually being used for display are the new "Diagnostic*" groups, so
linking the old "LspDiagnostics*" groups to these does absolutely
nothing, since there is nothing actually being highlighted with the
LspDiagnostics* groups.

These links were made in an attempt to preserve backward compatibility
with existing colorschemes. We could reverse the links to maintain this
preservation, but then that disallows us from actually defining default
values for the new highlight groups.

Instead, just remove the links and be done with the old LspDiagnostics*
highlight groups.

This is not technically a breaking change: the breaking change already
happened in #15585, but this PR just makes that explicit.

Use the 'default' keyword to prevent overriding existing highlight groups.

ref #15585

This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).

More discussion at [1].

[1]: https://github.com/neovim/neovim/pull/15585