Remove and Clarify netrw Options

[alex-courtis]

Eager disable of netrw is working: fb8735e

Remove the netrw options (with a message pointing to a wiki page) and associated codepaths.

See #1270

[alex-courtis]

https://github.com/nvim-tree/nvim-tree.lua/wiki/Tips#netrw-keeps-popping-up and :help should be enough.

Don't remove netrw functionality, as users do sometimes use other netrw functionality #1751