mime.lua: Obsolete require("io")

The `io` package is included but never used.

src/mime.lua

@@ -10,7 +10,6 @@
 local base = _G
 local ltn12 = require("ltn12")
 local mime = require("mime.core")
-local io = require("io")
 local string = require("string")
 local _M = mime
 
@@ -87,4 +86,4 @@ function _M.stuff()
     return ltn12.filter.cycle(_M.dot, 2)
 end
 
-return _M
+return _M