Mps br/timing: (*sigh*) have to back out everything (mpslib.h and

mpmst.h) to check it still builds, having rearranged
message functions into a consistent order.

Copied from Perforce
 Change: 166588
 ServerID: perforce.ravenbrook.com

mps/code/mpmst.h

@@ -223,7 +223,6 @@ typedef struct MessageStruct {
   MessageType type;             /* Message Type */
   MessageClass class;           /* Message Class Structure */
   RingStruct queueRing;         /* Message queue ring */
-  mps_clock_t postedClock;      /* mps_clock() at post time */
 } MessageStruct;