Remove extraneous logic

[mhashizume]

Every version of Ruby released for at least the last decade was set to use the -DRUBY19 flag. This commit sets the behavior in the flag as the default.

Description

I was attempting to build ruby-shadow on Ruby 4.0.0-preview2 and hit the same issue that's been worked around since Ruby 1.9.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• If Gemfile.lock has changed, I have used --conservative to do it and included the full output in the Description above.
• All new and existing tests passed.
• All commits have been signed-off for the Developer Certificate of Origin.