Remove global variable that disables dragging for all maps during zoom

Starting a zoom animation will disable dragging for all maps on the
page. Only the map that is being zoomed should have dragging disabled.

This fix removes a global static variable which is redundant since
the map pane already has a css class assigned to it for the duration of
the zoom animation.

Fixes #2539
Merge pull request #3050 from kdeloach/topic/disable-zoom-scrolling

Remove global variable that disables dragging for all maps during zoom