libaegisub: do not free string returned by getenv()

getenv() returns a string that is part of internal process data and
should never be modified and/or free()'d.

Originally committed to SVN as r5368.

aegisub/libaegisub/unix/path.cpp

@@ -42,7 +42,6 @@ const std::string home() {
 		exit(1);
 	}
 	std::string home(ehome);
-	free(ehome);
 	return home;
 }