Removed needless files from Gem::Specification#files

[hsbt]

The current implementation gives the following files by gem install net-smtp

.github/workflows/test.yml
.gitignore
Gemfile
LICENSE.txt
NEWS.md
README.md
Rakefile
bin/console
bin/setup
lib/net/smtp.rb
net-smtp.gemspec
test/net/fixtures/cacert.pem
test/net/fixtures/dhparams.pem
test/net/fixtures/server.crt
test/net/fixtures/server.key
test/net/smtp/test_response.rb
test/net/smtp/test_smtp.rb
test/net/smtp/test_ssl_socket.rb
test/net/smtp/test_sslcontext.rb
test/net/smtp/test_starttls.rb

I removed the needless files from them.