po: Drop prefixes from POTFILES.in

po: Drop prefixes from POTFILES.in

Commit 8beb7fdd0e23 changed the handling of POTFILES so that it
could cope with files being located in either the source or build
directory: it did so by adding @SRCDIR@ and @BUILDDIR@
respectively at the beginning of each line, and then converting
them back to the actual values when generating POTFILES from
POTFILES.in.

Later, commit c6a0d3ff8b4e started passing --directory to
xgettext, which resulted in the tool being able to locate files
regardless of whether they are in the source or build directory.
However, @SRCDIR@ and @BUILDDIR@ were still added to POTFILES.in
only to be stripped when generating POTFILES.

Simplify things by not storing information that we know we're
going to discard later.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>