LibWeb: Fix reset of floats y offset
Commit 7dc0edcb8625deae09317a1861d3c8d6ad9bca61 was supposed to prevent
floats from being placed higher than preceding boxes but the change
turned out to be completely wrong and caused regressions because:
1. Call to `clear()` in `layout_block_level_children` would reset
floating boxes only after layout of box with _not_ inline children
although the same should happen after layout of IFC.
2. `clear()` causes offset y of floats to be reset but it also clears
all currently enocuntered floating boxes which means the next box
that has actual clearance will get wrong y position.