Revert "backends/libinput: don't multiply v120 value by scroll speed"

This reverts commit 9cd52b40.

This commit broke the adjustable scroll speed feature on Wayland. That
feature may not have been implemented in the most technically ideal way,
but simply breaking it and leaving the mouse KCM with UI control that
does nothing isn't great. Besides, there was always a simple way to
avoid any scroll-related regressions caused by this feature: reset the
scroll speed to its default value, which internally is a multiplier of 1
and exercises the same codepath that 9cd52b40 imposes on everyone.

We can come up with a better implementation for Plasma 6.

BUG: 464592
FIXED-IN: 5.27.4