fix(rubocop): don't auto-detect bundler, to avoid surprising configuration behavior

[gongfarmer]

My earlier PR #2706 added an on_new_config handler which auto-detects whether bundler is used in a project, and calls rubocop through bundler or not based on the result.

This had some issues:

• may override the user's custom configuration due to the use of on_new_config
• has surprising side effect when overridden because the on_new_config handler runs twice
• incorrectly configures 'bundle exec rubocop --lsp' command if the project has a Gemfile that does not include rubocop

After some discussion it seems that this simpler configuration is best.
This PR deletes the on_new_config handler, leaving the lsp command to be rubocop --lsp.
This will also match the behaviour of the solargraph and ruby_ls configurations.

Users who want to use 'bundle exec rubocop --lsp` can get this with a simple custom lsp configuration, or by using a plugin such as https://github.com/mihyaeru21/nvim-lspconfig-bundler .

[janko]

I wanted to ensure that the Rubocop version in Gemfile.lock runs, because that's the version that the project uses. I also wanted to activate the Rubocop LSP only for projects that actually use Rubocop, so I chose to detect .rubocop.yml instead. I ended up with the following configuration:

-- init.lua
-- ...
local lspconfig = require("lspconfig")
lspconfig.rubocop.setup({
  cmd = { "bundle", "exec", "rubocop", "--lsp" },
  root_dir = lspconfig.util.root_pattern(".rubocop.yml"),
})