refactor!: make unsued util functions private

If any public function is not used by any configuration, then we can
simply make them private. It is possible that there are external uses of
these functions, but a preliminary search indicated that usage is small
to non-existent.

Functions that are privatized:
- get_active_clients_list_by_ft

Work on neovim#2079.

lua/lspconfig/util.lua

@@ -208,7 +208,7 @@ function M.insert_package_json(config_files, field, fname)
   return config_files
 end
 
-function M.get_active_clients_list_by_ft(filetype)
+local function get_active_clients_list_by_ft(filetype)
   local clients = M.get_lsp_clients()
   local clients_list = {}
   for _, client in pairs(clients) do
@@ -225,7 +225,7 @@ end
 
 function M.get_other_matching_providers(filetype)
   local configs = require 'lspconfig.configs'
-  local active_clients_list = M.get_active_clients_list_by_ft(filetype)
+  local active_clients_list = get_active_clients_list_by_ft(filetype)
   local other_matching_configs = {}
   for _, config in pairs(configs) do
     if not vim.tbl_contains(active_clients_list, config.name) then