Remove not needed and deprecated Boost.Signals header

`"boost/signals.hpp"` is not used in the package and since Boost 1.56.0 is
considered unmaintained and deprecated.

    warning: #warning "Boost.Signals is no longer being maintained and
    is now deprecated. Please switch to Boost.Signals2. To disable this
    warning message, define BOOST_SIGNALS_NO_DEPRECATION_WARNING." [-Wcpp]

The patch removes the header.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>