Remove unnecessary #ifdef FRONTEND check to choose between strdup and pstrdup.

The libpgcommon patch made that unnecessary, palloc and friends are now
available in frontend programs too, mapped to plain old malloc.

As pointed out by Alvaro Herrera.

src/port/wait_error.c

@@ -34,7 +34,6 @@ char *
 wait_result_to_str(int exitstatus)
 {
 	char		str[512];
-	char	   *result;
 
 	if (WIFEXITED(exitstatus))
 	{
@@ -83,10 +82,5 @@ wait_result_to_str(int exitstatus)
 				 _("child process exited with unrecognized status %d"),
 				  exitstatus);
 
-#ifndef FRONTEND
-	result = pstrdup(str);
-#else
-	result = strdup(str);
-#endif
-	return result;
+	return pstrdup(str);
 }