gh-40349: remove the auld historic "Group" class
    
This was deprecated in #37449

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40349
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/groups/meson.build

@@ -25,7 +25,6 @@ py.install_sources(
   'libgap_mixin.py',
   'libgap_morphism.py',
   'libgap_wrapper.pxd',
-  'old.pxd',
   'pari_group.py',
   'raag.py',
   subdir: 'sage/groups',
@@ -34,7 +33,6 @@ py.install_sources(
 extension_data = {
   'group' : files('group.pyx'),
   'libgap_wrapper' : files('libgap_wrapper.pyx'),
-  'old' : files('old.pyx'),
 }
 
 foreach name, pyx : extension_data