updater, minzip: Remove unnecessary O_SYNC flags.

Remove O_SYNC from mzExtractRecursive() and PackageExtractFileFn().
These functions deal with extracting whole files from the update
package onto a filesystem. If run on ext4 on a rotating disk, for
example, the O_SYNC flag will cause serious performance problems
and the extraction proecss can take over 30 minutes, with no
obvious benefits.

This API function already calls fsync(fd) after each file is
extracted to ensure data and metadata is written to the underlying
block device, so the O_SYNC calls should be superfluous and safely
removable.

This change does not affect the OTA patch paths or any modification
of the bootloader partition or writes to other 'emmc' partitions.

Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>
Change-Id: I9cbb98a98e6278bf5c0d7efaae340773d1fbfcd2