refactor: remove vim.g.oil declarative config

Problem: the `vim.g.oil` configuration path was added prematurely.
It adds a second config entrypoint before the plugin has stabilized
enough to justify it.

Solution: remove `vim.g.oil` support from `plugin/oil.lua`,
`config.setup()`, docs, and tests. Users configure via
`require("oil").setup({})`.

plugin/oil.lua

@@ -1,3 +1 @@
-if vim.g.oil ~= nil then
-  require('oil').setup()
-end
+