configure.ac: remove manual AC_SUBST for pthread-stubs

Unneeded, since the PKG_CHECK_MODULES macro already does the
substitution of the package Cflags/Libs.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

configure.ac

@@ -856,8 +856,6 @@ esac
 
 if test "x$pthread_stubs_possible" = xyes; then
     PKG_CHECK_MODULES(PTHREADSTUBS, pthread-stubs >= 0.4)
-    AC_SUBST(PTHREADSTUBS_CFLAGS)
-    AC_SUBST(PTHREADSTUBS_LIBS)
 fi
 
 dnl SELinux awareness.