[linux] Remove all traces of signalfd(2)

Summary:
Signalfd is not used in the code anymore, and given that the same functionality can be achieved
with the new MainLoop class, it's unlikely we will need it in the future. Remove all traces of
it.

Reviewers: tberghammer, ovyalov

Subscribers: tberghammer, danalbert, srhines, lldb-commits

Differential Revision: http://reviews.llvm.org/D17510

llvm-svn: 261631