Drop duplicate linking to libvncserver.

Summary:
The duplicate link may also be missing necessary -L flags,
so it causes linker errors when libvncserver is installed
in unusual places (while ${LIBVNCSERVER_LIBRARIES} DTRT).

Reviewers: sitter, apol, ltoscano, heikobecker

Reviewed By: sitter

Differential Revision: https://phabricator.kde.org/D5018