Remove stale dependency to libhwc2on1adaptor.so.

This commit removes stale dependency to libhwc2on1adaptor.  Since commit
831d48ed, mAdapter has become an unused private member.  In addition,
this unused unique_ptr leads to an unnecessary symbol reference to the
destructor of HWC2on1Adaptor.  This commit removes them completely.

Bug: 38302528
Test: Sailfish builds and boots.
Change-Id: I0208d7fb21ff2f5d59d367162bc5f982f1515982
Merged-In: I479f12ddbae91282a13464159f00513aa7cb3208