initrd: remove the BLKFLSBUF call in handle_initrd

BLKFLSBUF used to be overloaded for the ramdisk driver to free the whole
ramdisk, which was completely different behavior compared to all other
drivers.  But this magic overload got removed in commit ff269568
("brd: remove support for BLKFLSBUF"), so this call is entirely
pointless now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>