[FIX] website: properly revert slider on break unbreakable

When breaking a page from a Carousel or other slider snippet (eg by
hitting the backspace key at the beginning of a page), the editor's
history gets reverted but ends up applying the `active` class to all
pages. This is due is some way to the `_computeHeights` method of the
slider snippet, which applied the class to all pages to observe their
heights, then put them back in place. It seems in the labyrinth of ticks
the history step somehow started from a place where every page was
active, so we reverted back to that. The point where these classes were
applied should have never been observed in the first place since it's
purely technical. Not observing it fixes the bug.

task-3081259

closes odoo/odoo#106696

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>