Fix deleting columns in wxGridStringTable with custom column labels.

Fix deleting columns in wxGridStringTable with custom column labels.

We erroneously removed too many elements from m_colLabels array (basically we
always removed all the elements remaining after this column, irrespectively of
the actual number of columns to delete), fix this by removing at most the
specified number of columns -- or possibly less if the array isn't entirely
filled.

See #13329.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68306 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775