feature: Remove union tool retain pieces option

[DSCaskey]

This PR removes the option to Retain orginal pieces in the Union Tool. Removing the option means the tool can't delete the pieces.

closes issue #1215

[what-the-diff]

🎉 Fantastic Pull Request Summary 🌈

We're absolutely thrilled to share these refreshing changes with you. Here's a nice friendly version of the technical stuff!

• Language Simplification 🌎
  We've tidied up some translation entries related to the "Retain original pieces" feature in multiple languages (like Czech, German, Greek, English, Spanish, and others). This helps us maintain a cleaner and leaner codebase.

• Polishing up union_dialog.cpp 🛠️
  We've waved goodbye to the retainPieces() method. And in the spirit of providing the most inviting code, we've renamed the checkPiece() to isPieceValid(). It's all about making things clear and understandable!

• Interface Update for union_dialog.ui 🖥️
  Hang onto your hats! We have transformed the dialog's User Interface (UI) by removing the "Retain original pieces" checkbox. It's all part of our never-ending quest for a simpler, sleeker design!

Keep rocking guys! Each of your contributions brings us one step closer to our ultimate goals! Let's code for a better future, together. 🚀 🌍 💫