Remove `Op::getIndices()` (#8355)

Remove `Op::getIndices()` (#8355)

This commit removes `Op::getIndices()`. As discussed offline, the
semantics of that method were confusing and the use cases are covered by
`Op::getNumIndices()` and `Op::operator[]()` (which mirror
`Term::getNumChildren()` and `Term::operator[]()`).

Future changes are going to update the Python and Java bindings to
support `Op::operator[]()`.