refacto: deprecate used_by in parsers.lua

- remove print_warning function from utils.lua (unused)
- cleanup some functions in parsers.lua (parameters overloading
  and wrong bufnr used).
- log a deprecation notice when using used_by in a parser definition
- default the filetype_to_parsername table to the list of filetypes
  previously in the used_by keys
- update the README to indicate that change

lua/nvim-treesitter/utils.lua

@@ -136,12 +136,6 @@ function M.get_at_path(tbl, path)
   return result
 end
 
--- Prints a warning message
--- @param text the text message
-function M.print_warning(text)
-  api.nvim_command(string.format([[echohl WarningMsg | echo "%s" | echohl None]], text))
-end
-
 function M.set_jump()
   vim.cmd "normal! m'"
 end