Remove "Type=ext4" option in opt-usr.mount to support f2fs filesystem

Remove "Type=ext4" option in opt-usr.mount to support f2fs filesystem

The "Type=" option in opt-usr.mount should not be specified,
because tizen uses "ext4" or "f2fs" filesystem for each target.

If there is no "Type=" option in the mount unit,
systemd automatically detects it when mounting.

Reference.
 - https://www.freedesktop.org/software/systemd/man/systemd.mount.html
 - Type= : Takes a string for the file system type. See mount(8) for details. This setting is optional

Change-Id: I4127b6534c2d593a24d3cc3727a83b7101ad3740