Optimized Boost Build's generator selection related viable-source-types-real() rule. Removed an extra sequence.unique rule called on the resulting sequence that was not really needed as elements get added to the sequence only if they are not already in it.

[SVN r48587]

v2/build/generators.jam

@@ -755,7 +755,7 @@ local rule viable-source-types-real ( target-type )
         }
     }
 
-    return [ sequence.unique $(result) ] ;
+    return $(result) ;
 }