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/exit.c

@@ -41,7 +41,6 @@ void clear_proc(struct vmproc *vmp)
 	vmp->vm_region_top = 0;
 	vmp->vm_callback = NULL;	/* No pending vfs callback. */
 	vmp->vm_flags = 0;		/* Clear INUSE, so slot is free. */
-	vmp->vm_yielded = 0;
 #if VMSTATS
 	vmp->vm_bytecopies = 0;
 #endif