Removed glibc crypt_r() bug workaround (ticket #1469).

The bug in question was fixed in glibc 2.3.2 and is no longer expected
to manifest itself on real servers.  On the other hand, the workaround
causes compilation problems on various systems.  Previously, we've
already fixed the code to compile with musl libc (fd6fd02f6a4d), and
now it is broken on Fedora 28 where glibc's crypt library was replaced
by libxcrypt.  So the workaround was removed.

src/os/unix/ngx_user.c

@@ -21,10 +21,6 @@ ngx_libc_crypt(ngx_pool_t *pool, u_char *key, u_char *salt, u_char **encrypted)
     struct crypt_data   cd;
 
     cd.initialized = 0;
-#ifdef __GLIBC__
-    /* work around the glibc bug */
-    cd.current_salt[0] = ~salt[0];
-#endif
 
     value = crypt_r((char *) key, (char *) salt, &cd);