apt manpage is built from xml nowadays like the rest

It used be a handwritten manpage, but that is gone and this artifact is
the cause for the message:
../../buildlib/manpage.mak:23: target '../../build/docs/apt.de.8' given
more than once in the same rule
[ … repeated for all translations … ]
So lets get right of it.

Git-Dch: Ignore

buildlib/po4a_manpage.mak

@@ -57,7 +57,6 @@ HAVE_PO4A=yes
 endif
 
 # take care of the rest
-SOURCE := $(SOURCE) $(wildcard apt.$(LC).8)
 INCLUDES :=
 
 ifndef HAVE_PO4A