raw_istream and kin: a few cleanups.

anchor() doesn't seem to change compiler output in current compilers, so
remove it.  Use default where appropriate rather than empty bodies.

src/raw_socket_istream.cpp

@@ -9,8 +9,6 @@
 
 using namespace nt;
 
-raw_socket_istream::~raw_socket_istream() {}
-
 bool raw_socket_istream::read(void* data, std::size_t len) {
   char* cdata = static_cast<char*>(data);
   std::size_t pos = 0;