remove SvStream::operator>> methods

in favour of ReadXXX methods.

Change-Id: Ic2c0a7b6b92ff4c236ae99b39d77f3d935b301e3
Reviewed-on: https://gerrit.libreoffice.org/7915
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>