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/extra_plugins/__init__.py

@@ -2,8 +2,4 @@
 #
 # SPDX-License-Identifier: MPL-2.0
 
-"""Extra plugins. These are not automatically inserted when ocrmypdf is run.
-
-You can use these plugins by specifying them on the command line, e.g.:
-ocrmypdf --plugin ocrmypdf.extra_plugins.semfree ...
-"""
+"""Extra plugins. These are not automatically inserted when ocrmypdf is run."""