Remove scroll units duplication in wxGrid to fix setting them.

Remove scroll units duplication in wxGrid to fix setting them.

For some unknown reason wxGrid decided to store its scroll units in its own
m_scrollLine[XY] variables instead of just using the base wxScrollWindow class
m_[xy]ScrollPixelsPerLine ones. And, of course, the two could get out of sync
because wxGrid didn't update the base class version correctly.

Just don't duplicate these values at all and use the base class fields. This
makes the code simpler and also fixes changing the size of the scroll units.

Closes #12221.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64918 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775