staging: exfat: remove sync_alloc_bitmap()

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

Signed-off-by: Kaaira Gupta <kgupta@es.iitr.ac.in>
Link: https://lore.kernel.org/r/20200223192347.GA20286@kaaira-HP-Pavilion-Notebook


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