mime: Remove io dependency

This dependency was spuriously added, maybe for debug reasons,
as confirmed to me by Diego Nehab by mail.
Some systems based in Lua (e.g. Ginga) prohibit the use of
io module for security reasons, so this dependency makes
mime unusable; even worse this makes other modules, based
on mime, unusable too (e.g. html).

src/mime.lua

@@ -11,7 +11,6 @@
 local base = _G
 local ltn12 = require("ltn12")
 local mime = require("mime.core")
-local io = require("io")
 local string = require("string")
 module("mime")