Remove dead code

Dead since 919fa12, ie. v0.0.4

src/common.lua

     --
     local action = os.getenv("IMAPDOMO_ACTION")
     local file = os.getenv("IMAPFILTER_HOME") .. "/handlers/" .. action .. ".lua"
-    local valid = {
-        newmail = true,
-        rewind = true,
-        cleanup = true,
-        migrate = true
-    }
     if file_exists(file) then
         do_if_exists("init.lua")
         dofile(file)