gemspec: Remove unnecessary files from gem file

[daipom]

Which issue(s) this PR fixes:
None.

What this PR does / why we need it:
Remove the following files from the gem file:

• test files
• git files
• Gemfile

This reduces the gem file size: 867k -> 558k.

We should reflect recent gemspec moves as the current files and test_files are written more than 10 years ago:

• Goodbye jeweler and welcome bundler! #116

test_files is not documented anymore:

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

Looks like it is not recommended:

• Don't package test files with gem rubygems/bundler#3207

Looks like we should also remove git files and Gemfile because 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

The removed files except for test/* are as follows:

before = `git ls-files`.split($\)
after = Dir.chdir(__dir__) do
  `git ls-files -z`.split("\x0").reject do |f|
    (File.expand_path(f) == __FILE__) ||
      f.start_with?(*%w[test/ .git Gemfile])
  end
end
before.reject{|path| path.start_with? "test/"}.difference after
=>
[".github/DISCUSSION_TEMPLATE/q-a-japanese.yml",
 ".github/DISCUSSION_TEMPLATE/q-a.yml",
 ".github/ISSUE_TEMPLATE.md",
 ".github/ISSUE_TEMPLATE/bug_report.yml",
 ".github/ISSUE_TEMPLATE/config.yml",
 ".github/ISSUE_TEMPLATE/feature_request.yml",
 ".github/PULL_REQUEST_TEMPLATE.md",
 ".github/workflows/stale-actions.yml",
 ".github/workflows/test-ruby-head.yml",
 ".github/workflows/test.yml",
 ".gitignore",
 "Gemfile"]

after.difference before
=> []

Docs Changes:
Not needed.

Release Note:
Not needed.

[ashie]

LGTM
Thanks!