Don't apply Rust function snippet logic if rust-analyzer is used

[flodiebold]

rust-analyzer has better server-side logic, and in fact this fallback logic only applied when it was undesirable (e.g. when importing functions), because that's when RA doesn't return a snippet.

[sebastiencs]

The function company-lsp--rust-completion-snippet is outdated. It was implemented for RLS but now even RLS provides snippets. So I think company-lsp--rust-completion-snippet can be removed from company-lsp.