sendfile: retire SF_SYNC

The flag was added in b75a1171d8ba4 for the sake of Varnish. However, that
idea didn't work. Quoting email from Poul-Henning:

"We had to give up sendfile(), the overhead of keeping track of everything
between the network stack and VM system made ate any gain we saw. ... I
dont think we ever shipped a version where sendfile was enabled by
default."

PR: 287348