nvim: remove `once' helper

.config/nvim/lua/once.lua

@@ -1,15 +0,0 @@
--- A helper that prevents something to be loaded more than once
-M = {}
-
----@param label string
----@param callback function
-function M.test_and_load(label, callback)
-    local once = vim.g.once or {}
-    if not once[label] then
-        callback()
-        once[label] = true
-        vim.g.once = once
-    end
-end
-
-return M