[orc-rt] Remove (effectively) unused header orc-rt-c/orc-rt.h. NFCI.

This header was a placeholder in the initial project check-in, but is not used.
Time to remove it.

Author: lhames

orc-rt/include/orc-rt-c/orc-rt.h

@@ -1,5 +1,4 @@
 set(files
-  orc-rt-executor.cpp
   AllocAction.cpp
   ResourceManager.cpp
   RTTI.cpp

orc-rt/lib/executor/CMakeLists.txt

@@ -10,9 +10,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "orc-rt-c/orc-rt.h"
-
 int main(int argc, char *argv[]) {
-  orc_rt();
   return 0;
 }