fix: remove copilot provider (#318)

lua/avante/utils/tokens.lua

@@ -1,6 +1,4 @@
 --Taken from https://github.com/jackMort/ChatGPT.nvim/blob/main/lua/chatgpt/flows/chat/tokens.lua
-local Tiktoken = require("avante.tiktoken")
-
 ---@class avante.utils.tokens
 local Tokens = {}
 
@@ -13,9 +11,6 @@ local cost_per_token = {
 ---@param text string The text to calculate the number of tokens for.
 ---@return integer The number of tokens in the given text.
 function Tokens.calculate_tokens(text)
-  if Tiktoken.available() then
-    return Tiktoken.count(text)
-  end
   local tokens = 0
   local current_token = ""
   for char in text:gmatch(".") do