Remove Signals from doc/pdf/Jamfile.v2

doc/pdf/Jamfile.v2

@@ -77,7 +77,6 @@ install pdf-install
     ../../libs/ratio/doc//standalone
     ../../libs/random/doc//standalone
     ../../libs/regex/doc//standalone
-    ../../libs/signals/doc//doc
     ../../libs/spirit/repository/doc//spirit2_repository
     ../../libs/static_assert/doc//standalone
     ../../libs/thread/doc//standalone