extensions: remove call to gettext.bind_textdomain_codeset().

I added this call originally, mimicking the C plug-ins. Yet in Python,
this function is deprecated since Python 3.8 and removed in 3.10.
Looking closer in the docs, it looks like Python's gettext module works
differently.

While all the *gettext() functions return strings in the locale's
encoding in C, unless overriden by bind_textdomain_codeset() (therefore
we need this call, in order to have locale-independent string encoding),
this is not the case in Python, whose gettext() call is already
locale-independent. The gettext.bind_textdomain_codeset() function only
applies to the specific l*gettext() functions which are deprecated too.

Fixes the warning:

> goat-exercise-py3.py:37: DeprecationWarning: bind_textdomain_codeset() is deprecated

extensions/goat-exercises/goat-exercise-py3.py

@@ -34,7 +34,6 @@ import sys
 # whereas the below calls are used for localization within the plug-in.
 textdomain = 'gimp30-std-plug-ins'
 gettext.bindtextdomain(textdomain, Gimp.locale_directory())
-gettext.bind_textdomain_codeset(textdomain, 'UTF-8')
 gettext.textdomain(textdomain)
 _ = gettext.gettext
 def N_(message): return message