Remove mps_arena_expose, deprecated since release 1.111.0

Also remove mps_arena_unsafe_expose_remember_protection and
mps_arena_unsafe_expose_restore_protection.

mps/code/mpmst.h

@@ -534,12 +534,6 @@ typedef struct GlobalsStruct {
   RingStruct rootRing;          /* ring of roots attached to arena */
   Serial rootSerial;            /* serial of next root */
 
-  /* remember summary <code/trace.c> */
-  RingStruct rememberedSummaryRing;
-  /* index into next free slot in block.  0 means that a new
-     block should be allocated and appended. */
-  Index rememberedSummaryIndex;
-
   /* locus <code/locus.c> */
   Chain defaultChain;           /* default chain for GC pool */
 } GlobalsStruct;