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({})`.

doc/oil.txt

@@ -48,25 +48,12 @@ REQUIREMENTS                                                    *oil-requirement
 --------------------------------------------------------------------------------
 CONFIG                                                                *oil-config*
 
-Oil can be configured in two ways:
-
-1. The traditional `setup()` call: >lua
+Configure oil via `setup()`: >lua
     require("oil").setup({
       -- your opts here
     })
 <
 
-2. Declarative configuration via `vim.g.oil`, which does not require calling
-   `setup()`: >lua
-    vim.g.oil = {
-      -- your opts here
-    }
-<
-
-   When `vim.g.oil` is set, oil initializes automatically during plugin
-   loading. If `setup()` is called with explicit opts, those take precedence
-   over `vim.g.oil`.
-
 The full list of options with their defaults:
 
 >lua