tests: remove useless filter for coverage tests

The filter for `vendor/bundle/` is useless because this directory is
located in `Homebrew/test/` and that one is already filtered. Moreover,
SimpleCov already loads the `bundler_filter` profile (that installs
basically the same filter) in its default configuration.

Library/Homebrew/test/.simplecov

@@ -7,7 +7,6 @@ SimpleCov.start do
   coverage_dir File.expand_path("#{tests_path}/coverage")
   root File.expand_path("#{tests_path}/../../")
 
-  add_filter "vendor/bundle/"
   add_filter "Formula/"
   add_filter "Homebrew/compat/"
   add_filter "Homebrew/test/"