Makefile: Remove POT-Creation-Date from circuitpython.pot

This line is only a magnet for conflicts; the date that circuitpython.pot
was modified can be extracted from git metadata.  Additionally, when
we add "make translate" checking to pre-commit, this will avoid spurious
changes since the most straightforward implementation would otherwise
update this line every time pre-commit executed.

Author: jepler

Makefile

@@ -222,7 +222,7 @@ pseudoxml:
 all-source:
 
 locale/circuitpython.pot: all-source
-	find $(TRANSLATE_SOURCES) -type d \( $(TRANSLATE_SOURCES_EXC) \) -prune -o -type f \( -iname "*.c" -o -iname "*.h" \) -print | (LC_ALL=C sort) | xgettext -f- -L C -s --add-location=file --keyword=translate -o circuitpython.pot -p locale
+	find $(TRANSLATE_SOURCES) -type d \( $(TRANSLATE_SOURCES_EXC) \) -prune -o -type f \( -iname "*.c" -o -iname "*.h" \) -print | (LC_ALL=C sort) | xgettext -f- -L C -s --add-location=file --keyword=translate -o - | sed -e '/"POT-Creation-Date: /d' > $@
 
 # Historically, `make translate` updated the .pot file and ran msgmerge.
 # However, this was a frequent source of merge conflicts.  Weblate can perform