Remove misc.readonly

Fix #571

lua/cmp/utils/misc.lua

@@ -168,19 +168,6 @@ misc.to_vimindex = function(text, utfindex)
   return utfindex + 1
 end
 
----Return readonly version object
----@generic T
----@param tbl T
----@return T
-misc.readonly = function(tbl)
-  return setmetatable({}, {
-    __index = tbl,
-    __newindex = function()
-      error('this table is readonly.')
-    end,
-  })
-end
-
 ---Mark the function as deprecated
 misc.deprecated = function(fn, msg)
   local printed = false