fs: Don't complain about failing to read /proc/self/exe with Boost.filesystem

This is intended to silently fail on Unix-like platforms where no procfs
is available, and indeed that's how it works in the non-BFS
implementation of get_exe_dir(). The caller (currently a single function
in game.cpp/wesnoth.cpp) must be able to deal with this situation
in a graceful fashion.

src/filesystem_boost.cpp

@@ -637,7 +637,6 @@ std::string get_exe_dir()
 	error_code ec;
 	path exe = bfs::read_symlink(self_exe, ec);
 	if (ec) {
-		ERR_FS << "Failed to dereference " << self_exe.string() << ": " << ec.message() << '\n';
 		return std::string();
 	}