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 applies to everyone.
We can come up with a better implementation for Plasma 6.
BUG: 464592 FIXED-IN: 5.27.4