self: Remove reference to source.

A regression was introduced in 368e58afcb which
led to the guix-translated-texinfo derivation including a source file for the
entire checkout.

I'm not sure what the --srcdir option does, but the derivation still seems to
build when it's removed.

* guix/self.scm (translate-texi-manuals): Remove #$source reference.

Change-Id: Icfe1eaf7600f5708c9b3408d59d760b1edb3237b

guix/self.scm

@@ -324,7 +324,6 @@ the result to OUTPUT."
               "--variable" (string-append "master=" source)
               "--variable" (string-append "po=" po)
               "--variable" (string-append "pot=" (string-append (tmpnam) ".pot"))
-              (string-append "--srcdir=" #$source)
               "--destdir=."
               #+(file-append documentation-po "/po4a.cfg")))