Removes unused signal handlers left over from signal based collection
and block the non windows-relevant collectors loadavg and interrupts.
Signal based collection removed in 1c17481a42.
As OS X doesn't have it's own interrupts provider, don't build
interrupts_common on OS X as well. Otherwise build fails, because
interrupts_common depends on variables provided by platform-specific
files.
Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>
