Remove an unneeded cpu_dcache_wb_range from arm64

The cpu_dcache_wb_range function is an expensive function that is
unneeded in ddb. It is used when the cache needs to be written to RAM,
e.g. when working with a non-cache coherent device.

Remove it as cpu_icache_sync_range already has the needed d-cache
handling to ensure any changed memory is visible to the i-cache.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37037