[libFuzzer] Remove InterruptHandler from Fuchsia implementation

As implemented, the `InterruptHandler` thread was spinning trying to
`select()` on a null "stdin", wasting a significant amount of CPU for no
benefit. As Fuchsia does not have a native concept of stdin (or POSIX
signals), this commit simply removes this feature entirely.

Reviewed By: aarongreen

Differential Revision: https://reviews.llvm.org/D89266