[rubygems/rubygems] Remove bad `bundle install`

This spec is specifically testing for the case where there's no
`Gemfile.lock` file and it's only doing the expected thing because the
`bundle install` command is silently failing. Remove the `bundle
install` to reduce confusion.

https://github.com/rubygems/rubygems/commit/ec39fbde0e

spec/bundler/commands/update_spec.rb

@@ -694,7 +694,7 @@ RSpec.describe "bundle update" do
   end
 
   it "shows error message when Gemfile.lock is not preset and gem is specified" do
-    install_gemfile <<-G
+    gemfile <<-G
       source "#{file_uri_for(gem_repo2)}"
       gem "activesupport"
     G