Remove confusion over ambiguous symbols

libboost and libsigc++ both contain symbols called _1() / _2() etc. Forcing the use of namespace sigc causes us to use the ones from sigc++ when in fact, we want the ones from boost.

Hopefully this fix will work for all compilers.

libs/surfaces/osc/osc_controllable.cc

@@ -27,7 +27,6 @@
 #include "osc.h"
 #include "osc_controllable.h"
 
-using namespace sigc;
 using namespace PBD;
 using namespace ARDOUR;
 using namespace ArdourSurface;