plugin-generator: update gemspec to remove unnecessary files

We should reflect recent gemspec moves.

`test_files` is not documented anymore:

* https://guides.rubygems.org/specification-reference

The template of Bundler 2.4.13 is as follows:

    spec.files = Dir.chdir(__dir__) do
      `git ls-files -z`.split("\x0").reject do |f|
        (File.expand_path(f) == __FILE__) ||
          f.start_with?(*%w[bin/ test/ spec/ features/ .git .circleci appveyor Gemfile])
      end
    end

Signed-off-by: Daijiro Fukuda <[email protected]>

templates/new_gem/fluent-plugin.gemspec.erb

@@ -12,12 +12,13 @@ Gem::Specification.new do |spec|
   spec.homepage      = "TODO: Put your gem's website or public repo URL here."
   spec.license       = "<%= @license.name %>"
 
-  test_files, files  = `git ls-files -z`.split("\x0").partition do |f|
-    f.match(%r{^(test|spec|features)/})
+  spec.files         = Dir.chdir(__dir__) do
+    `git ls-files -z`.split("\x0").reject do |f|
+      (File.expand_path(f) == __FILE__) ||
+        f.start_with?(*%w[test/ spec/ features/ .git .circleci appveyor Gemfile])
+    end
   end
-  spec.files         = files
-  spec.executables   = files.grep(%r{^bin/}) { |f| File.basename(f) }
-  spec.test_files    = test_files
+  spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
   spec.add_development_dependency "bundler", "~> <%= bundler_version %>"