devel/qt5: avoid POST-DEINSTALL error messages

The deinstall script needs to check if QtCore/qconfig.h exists.
If it doesn't, then Qt as a whole is being deinstalled. However,
if it *does* exist, then the shell command [ ! -e QtCore/qconfig.h ]
exits with a non-zero exit code. In the deinstall script, that
was the last pipeline in the script and so the script ends up with
that as exit code. Suppress this bogus failure by adding : (true)
to the end of the deinstall script.

I'm specifically not bumping any PORTREVISIONs. The deinstall
message is harmless (if confusing) and rebuilding all the Qt packages
for a one-character change to suppress a harmless message seems
like overkill. They will doubtless be rebuilt soon for some
dependency change anyway.