Move Assert() definitions to c.h

This way, they can be used by frontend and backend code.  We already
supported that, but doing it this way allows us to mix true frontend
files with backend files compiled in frontend environment.

Author: Andres Freund

src/include/postgres_fe.h

@@ -24,16 +24,4 @@
 
 #include "c.h"
 
-/*
- * Assert() can be used in both frontend and backend code. In frontend code it
- * just calls the standard assert, if it's available. If use of assertions is
- * not configured, it does nothing.
- */
-#ifdef USE_ASSERT_CHECKING
-#include <assert.h>
-#define Assert(p) assert(p)
-#else
-#define Assert(p)
-#endif
-
 #endif   /* POSTGRES_FE_H */