ci: gentoo: don't accept unstable python-exec

Currently what this does is permit freethreaded builds, which are
stablemasked but not masked, which means we get a binpackage cache miss
for something we frankly very much do not care about at all.

We can and do unmask implementations themselves later on in this file,
which should handle all this for us anyway.

ci/ciimage/gentoo/install.sh

@@ -98,7 +98,6 @@ printf "%s\n" ${pkgs_latest[@]} >> /etc/portage/package.accept_keywords/meson
 cat /etc/portage/package.accept_keywords/meson
 
 cat <<-EOF > /etc/portage/package.accept_keywords/misc
-	dev-lang/python-exec
 	dev-lang/python
 EOF