Remove unused SEGMENT_PROT_RWX

It's been unused for a year and is problematic on any OS which
requires W^X for security.

rts/LinkerInternals.h

@@ -74,12 +74,10 @@ typedef enum {
     SEGMENT_PROT_RO  = PROT_READ,
     SEGMENT_PROT_RX  = PROT_READ | PROT_EXEC,
     SEGMENT_PROT_RWO = PROT_READ | PROT_WRITE,
-    SEGMENT_PROT_RWX = PROT_READ | PROT_WRITE | PROT_EXEC
 #else
     SEGMENT_PROT_RO,
     SEGMENT_PROT_RX,
     SEGMENT_PROT_RWO,
-    SEGMENT_PROT_RWX
 #endif
 } SegmentProt;