arm64: Stop setting VM_BCACHE_SIZE_MAX

This setting places a (small) limit on the size of the buffer cache,
constraining UFS performance on large servers.  The setting comes from
the initial arm64 implementation and appears to be vestigal.  Remove it.

Reviewed by:	kib
Submitted by:	Klara, Inc.
Sponsored by:	Ampere Computing
Differential Revision:	https://reviews.freebsd.org/D28162

sys/arm64/include/param.h

@@ -108,15 +108,6 @@
 #define	KSTACK_GUARD_PAGES	1	/* pages of kstack guard; 0 disables */
 #define	PCPU_PAGES		1
 
-/*
- * Ceiling on size of buffer cache (really only effects write queueing,
- * the VM page cache is not effected), can be changed via
- * the kern.maxbcache /boot/loader.conf variable.
- */
-#ifndef VM_BCACHE_SIZE_MAX
-#define	VM_BCACHE_SIZE_MAX      (400 * 1024 * 1024)
-#endif
-
 /*
  * Mach derived conversion macros
  */