[breaking change] remove direct reference to fastgit

fastgit may be blocked by GFW (see https://t.me/fastgitchannel/81), so
hard-coding its URL is not a good option. Using git URL rewrite feature
is a better choice: https://jdhao.github.io/2021/11/27/git_notes_s4/#use-a-git-mirror-site-for-cloning.

This is a breaking change, since previously I use hub.fastgit.org as the
url, now the url will be default github.com unless the user change it
using the above mentioned method to change the cloning URL.

lua/plugins.lua

@@ -4,14 +4,7 @@ local fn = vim.fn
 vim.g.package_home = fn.stdpath("data") .. "/site/pack/packer/"
 local packer_install_dir = vim.g.package_home .. "/opt/packer.nvim"
 
-local plug_url_format = ""
-if vim.g.is_linux then
-  plug_url_format = "https://hub.fastgit.org/%s"
-else
-  plug_url_format = "https://github.com/%s"
-end
-
-local packer_repo = string.format(plug_url_format, "wbthomason/packer.nvim")
+local packer_repo = "https://github.com/wbthomason/packer.nvim"
 local install_cmd = string.format("10split |term git clone --depth=1 %s %s", packer_repo, packer_install_dir)
 
 -- Auto-install packer in case it hasn't been installed.
@@ -365,9 +358,6 @@ require("packer").startup({
   config = {
     max_jobs = 16,
     compile_path = util.join_paths(vim.fn.stdpath('config'), 'lua', 'packer_compiled.lua'),
-    git = {
-      default_url_format = plug_url_format,
-    },
   },
 })