fix(lsp): revert change to buf.clear_references() #24238

Problem: in #24046 the signature of buf.clear_references() changed, which
indirectly breaks callers that were passing "ignored" args.

Solution: because util.buf_clear_references() already defaulted to "current buffer",
the change to buf.clear_references() isn't actually needed, so just revert it.

runtime/lua/vim/lsp/util.lua

@@ -1769,9 +1769,9 @@ do --[[ References ]]
 
   --- Removes document highlights from a buffer.
   ---
-  ---@param bufnr integer Buffer id
+  ---@param bufnr integer|nil Buffer id
   function M.buf_clear_references(bufnr)
-    validate({ bufnr = { bufnr, { 'n', 'nil' }, true } })
+    validate({ bufnr = { bufnr, { 'n' }, true } })
     api.nvim_buf_clear_namespace(bufnr or 0, reference_ns, 0, -1)
   end