Remove dependency on tree-sitter-nickel

[Xophmeister]

tree-sitter-nickel is a declared dependency in Cargo.toml, but it is only used in the [little-used] benchmark testing (topiary-core/benches/benchmark.rs). This benchmark was written before dynamic loading was implemented and, while we don't want to benchmark the dynamic loading -- as it can involve a download-and-build step -- we could still make use of this mechanism.

This would remove the dependency on tree-sitter-nickel and make topiary-config/languages.ncl the authoritative source.

(For a similar discussion for tree-sitter-json, see #1023.)