Remove match_checksum matcher

These code are obsoleted.

```ruby
describe file('/etc/services') do
  it { should match_md5checksum '35435ea447c19f0ea5ef971837ab9ced' }
end

describe file('/etc/services') do
  it { should match_sha256checksum '0c3feee1353a8459f8c7d84885e6bc602ef853751ffdbce3e3b6dfa1d345fc7a' }
end
```

Instead you can write like these.

```ruby
describe file('/etc/services') do
  its(:md5sum) { should eq '35435ea447c19f0ea5ef971837ab9ced' }
end

describe file('/etc/services') do
  its(:sha256sum) { should eq '0c3feee1353a8459f8c7d84885e6bc602ef853751ffdbce3e3b6dfa1d345fc7a' }
end
```