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

sys/arm64/arm64/db_interface.c

@@ -167,8 +167,10 @@ db_write_bytes(vm_offset_t addr, size_t size, char *data)
 				*dst++ = *data++;
 			dsb(ish);
 
-			/* Clean D-cache and invalidate I-cache */
-			cpu_dcache_wb_range(addr, (vm_size_t)size);
+			/*
+			 * Ensure the I & D cache are in sync if we wrote
+			 * to executable memory.
+			 */
 			cpu_icache_sync_range(addr, (vm_size_t)size);
 		}
 	}