Remove assert from wxStringOperationsUtf8::DecIter().

The assert prevented decrementing end() iterator, which is a valid
operation.

Fixes #15566.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74976 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775