Remove superfluous mouse capturing in wxGrid row/column labels windows.

Remove superfluous mouse capturing in wxGrid row/column labels windows.

This code recaptured the mouse in the window which had already had it, thus
triggering the (recently added, see r74677) asserts in CaptureMouse(). It also
didn't preserve the invariant of m_winCapture always having the capture. And
it was apparently completely unnecessary too.

So simply remove it to fix the asserts while drag-resizing wxGrid rows/columns.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74752 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775