vm: remove secondary cache code

This commit removes the secondary cache code implementation from
VM and its usage from libminixfs. It is to be replaced by a new
implementation.

Change-Id: I8fa3af06330e7604c7e0dd4cbe39d3ce353a05b1

servers/vm/Makefile

@@ -4,7 +4,7 @@
 PROG=	vm
 SRCS=	main.c alloc.c utility.c exit.c fork.c break.c \
 	mmap.c slaballoc.c region.c pagefaults.c \
-	rs.c queryexit.c yieldedavl.c regionavl.c pb.c \
+	rs.c queryexit.c pb.c regionavl.c \
 	mem_anon.c mem_directphys.c mem_anon_contig.c mem_shared.c
 
 .if ${MACHINE_ARCH} == "earm"