Remove custom wagtail page form

THe older page form allowed to change segment settings inline but since
support for this was temporarily removed in an earlier commit we can
clean this up too

src/wagtail_personalisation/forms.py

@@ -1,43 +0,0 @@
-from wagtail.wagtailadmin.forms import WagtailAdminPageForm
-
-
-class AdminPersonalisablePageForm(WagtailAdminPageForm):
-    """The personalisable page form that allows creation of variants."""
-
-    def save(self, commit=True):
-        """Save a copy of the original page, linked to a segment.
-
-        :returns: The original page, or a new page.
-        :rtype: wagtail_personalisation.models.PersonalisablePage
-
-        """
-        page = super(AdminPersonalisablePageForm, self).save(commit=False)
-
-        if page.segment:
-            segment = page.segment
-            slug = "{}-{}".format(page.slug, segment.encoded_name())
-            title = "{} ({})".format(page.title, segment.name)
-            update_attrs = {
-                'title': title,
-                'slug': slug,
-                'segment': segment,
-                'live': False,
-                'canonical_page': page,
-                'is_segmented': True,
-            }
-
-            if page.is_segmented:
-                slug = "{}-{}".format(
-                    page.canonical_page.slug, segment.encoded_name())
-                title = "{} ({})".format(
-                    page.canonical_page.title, segment.name)
-                page.slug = slug
-                page.title = title
-                page.save()
-                return page
-            else:
-                new_page = page.copy(
-                    update_attrs=update_attrs, copy_revisions=False)
-                return new_page
-
-        return page