Remove two useless declarations.

One quite recent - an unused variable in the Windows code that was
obsoleted by commit cea1329b9 last month - and one not recent at all,
namely the obsolete declaration of begin_session() in putty.h that
hasn't existed since commit 7a79df8fe replaced it with the ldisc
system in *2001*!

windows/winnpc.c

@@ -23,7 +23,6 @@ Socket *new_named_pipe_client(const char *pipename, Plug *plug)
     HANDLE pipehandle;
     PSID usersid, pipeowner;
     PSECURITY_DESCRIPTOR psd;
-    Socket *ret;
 
     assert(strncmp(pipename, "\\\\.\\pipe\\", 9) == 0);
     assert(strchr(pipename + 9, '\\') == NULL);