bsd.qt.mk: Remove qt5-webkit from devel/qt5/distinfo.

Since r422306, we use a separate distinfo file for www/qt5-webkit because it is
no longer part of official Qt5 releases. However, we forgot to remove it from
QT_DIST in bsd.qt.mk, which means `make makesum' from other Qt5 ports was
trying to fetch qt5-webkit from the wrong location.

Reviewed by: tcberner