Simplify and fix column determination code in wxGridStringTable.

The number of columns in the table is now (since r60717) stored in m_numCols,
so there is no need to get it from wxGrid any more. This is not only simpler
and faster but also more correct as the first row might not have the full
number of values yet.

Closes #15809.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75551 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775