Remove dead assignment in login(1)

[Frederic Cambus]

Hi tech@,

Since rev 1.36, the instance variable is never read again so we can
simply drop the else clause with the assignment.

Comments? OK?

Index: usr.bin/login/login.c
===================================================================
RCS file: /cvs/src/usr.bin/login/login.c,v
retrieving revision 1.69
diff -u -p -r1.69 login.c
--- usr.bin/login/login.c       8 Dec 2017 17:04:15 -0000       1.69
+++ usr.bin/login/login.c       7 Aug 2018 13:47:00 -0000
@@ -406,8 +406,7 @@ main(int argc, char *argv[])
                        if (strncmp(instance + 1, "root", 4) == 0)
                                rootlogin = 1;
                        *instance++ = '\0';
-               } else
-                       instance = "";
+               }
 
                if (strlen(username) > UT_NAMESIZE)
                        username[UT_NAMESIZE] = '\0';