SignalHandler: remove unused clear() function

It's not called, so not instantiated, so we don't know whether it
works at all. Remove it.

Amends acf7f0b1ae956f2fac7182c194e0441cd9c6f4d0 (5.4).

Let's just try to pick it back as far as possible. The compiler will
tell us when we pick into a branch where there's still a user, and we
can abandon that pick, then.

Pick-to: 6.11 6.10 6.8
Change-Id: I64ade5cbbe9785d50f7266b805d352b22c655fc2
Reviewed-by: Arno Rehn <a.rehn@menlosystems.com>