man: Source format 1.0 does not support building with upstream signatures

Building format 1.0 source packages with upstream signatures was
disabled in commit 92ca32c34f39f011c993e11746170c0258efb541, as a
feature that got introduced too suddenly. Update the documentation
in dpkg-source(1) to reflect that fact.

debian/changelog

@@ -20,6 +20,8 @@ dpkg (1.18.10) UNRELEASED; urgency=medium
       Prompted by Valentin Samir <valentin.samir@crans.org> in #830989.
     - Fix misspelled -fdebug-prefix-map flag in dpkg-buildflags(1).
       Reported by Mattia Rizzolo <mattia@debian.org>.
+    - Remove mention of source format 1.0 supporting building with upstream
+      tarball signatures from dpkg-source(1), as this was disabled in 1.18.8.
 
   [ Updated programs translations ]
   * German (Sven Joachim).

man/dpkg-source.1

@@ -292,7 +292,7 @@ A source package in this format consists either of a \fB.orig.tar.gz\fP
 associated to a \fB.diff.gz\fP or a single \fB.tar.gz\fP (in that case
 the package is said to be \fInative\fP).
 Optionally the original tarball might be accompanied by a detached
-upstream signature \fB.orig.tar.gz.asc\fP, extraction and building
+upstream signature \fB.orig.tar.gz.asc\fP, extraction
 supported since dpkg 1.18.5.
 
 .B Extracting