[API CHANGE] Remove .link feature from bootstrap variables

This feature had never been documented in include/rtl/bootstrap.h and had only
been used internally in LO on Windows for the multi-layer installations (thus is
unused since 827430c8c0417396b3c1d2a049ccddb818c89646 "Fold URE: Windows"), so
it should not hurt to remove it even if it could be considered part of the
stable URE interface.

Change-Id: If0f59f30dad8b2c3ff5f7931c19cd254073ccf80