remove redundant sync calls

The localmount and mount-ro scripts were flushing pending disk writes by
calling sync twice in succession. This is no longer necessary; see the
bug report and blog post for reasons we were still doing this.

Reported-by: Patrick Lauer <patrick@gentoo.org>
X-Gentoo-Bug: 487382
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=487382