scripts: sd_fusing_xu4: remove not necessary tmp variable and comment

scripts: sd_fusing_xu4: remove not necessary tmp variable and comment

Remove not necessary tmp variable and comment.

Note: it is partial revert of the commit e560d71f0d64 ("scripts:
odroid-xu4: add ramdisk.img flashing support") to flash ramdisk.img
at the last time. Almost all of revert is done with the commit
de355221bf93 ("scripts: sd_fusing_xu4: add the ramdisk partition")
but there was remaining variable and comment.

Change-Id: I1b28fa8ab5852fefa1d83b68b498e89ddafe5660
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>