sig_suspend_threads(): remove 'sending' arg

The TDA_AST flag is set on td2 unconditionally (as it was TDF_ASTPENDING
before AST rework), so it is not used practically for some time.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36033