Deprecate semfree and don't auto activate it

Instead the standard executor will fall back to threads.

semfree caused test failures  with Py3.14:
https://github.com/ocrmypdf/OCRmyPDF/issues/1558

In retrospect and with emerging Python tech like freethreading, semfree is becoming less necessary. We can use threads for the time being.

A consequence is that performance may be lower on Lambda and Termux when we are using threads and not shelling out work.

src/ocrmypdf/_plugin_manager.py

@@ -73,19 +73,10 @@ class OcrmypdfPluginManager(pluggy.PluginManager):
                 module = importlib.import_module(name)
                 self.register(module)
 
-        # 2. Install semfree if needed
-        try:
-            # pylint: disable=import-outside-toplevel
-            from multiprocessing.synchronize import SemLock
-
-            del SemLock
-        except ImportError:
-            self.register(importlib.import_module('ocrmypdf.extra_plugins.semfree'))
-
-        # 3. Register setuptools plugins
+        # 2. Register setuptools plugins
         self.load_setuptools_entrypoints('ocrmypdf')
 
-        # 4. Register plugins specified on command line
+        # 3. Register plugins specified on command line
         for name in self.__plugins:
             if isinstance(name, Path) or name.endswith('.py'):
                 # Import by filename