staging: exfat: remove exfat_buf_sync()

exfat_buf_sync() is not called anywhere, hence remove it from
exfat_cache.c and exfat.h

Signed-off-by: Kaaira Gupta <kgupta@es.iitr.ac.in>
Link: https://lore.kernel.org/r/20200223191623.GA20122@kaaira-HP-Pavilion-Notebook


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>