Fix delta calculation in mouswheel handler

Remove division by 10 of deltas, which makes scrolling slower than in previous versions.
Reverts 88d286957766e981a4f4c720f213164fc0f3f099

Tested with IE11, Firefox 27 and Google Chrome 33 - 35.

I don't know why this division was introduced, also couldn't find a hint on the jquery-mousewheel changelog that this is necessary. Also see examples/options-wheelSpeed.html - 'Slow: wheelSpeed:1' doesn't scroll at all if the delta values are divided by 10.