nvim: init: Drop exrc and secure

exrc is deprecated and not recommended. Accordingly also drop secure.

See
https://github.com/neovim/neovim/issues/5784#issuecomment-506993689

nvim/.config/nvim/init.lua

@@ -4,8 +4,6 @@ vim.o.scrolloff      = 3
 vim.o.sidescrolloff  = 5
 vim.o.textwidth      = 78
 vim.o.linespace      = 0
-vim.o.exrc           = true
-vim.o.secure         = true
 vim.o.showcmd        = true
 vim.o.showmatch      = true
 vim.o.ruler          = true