Remove unused rust-analyzer/syntaxTree

[stefnotch]

This seems to be a leftover after #18813

rust-analyzer/editors/code/src/lsp_ext.ts

Lines 49 to 51 in 4bf516e

	export const syntaxTree = new lc.RequestType<SyntaxTreeParams, string, void>(
	"rust-analyzer/syntaxTree",
	);

I suppose getting rid of it would be in order? It has already been removed from the documentation