deploy: Drop fsync of modified config files

deploy: Drop fsync of modified config files

These fsyncs were added for what turned out to be a fairly bogus
reason; I was hitting read errors from extlinux after upgrades and out
of conservatisim tried adding fsync calls, but the *actual* problem
was that extlinux didn't support 64 bit ext4.  Now that at least for
Project Atomic hosts we're just targeting grub2, we can drop these
fsync calls and rely on `syncfs()` being both faster and catching any
errors.