sesman: Remove unused authentication methods

The Kerberos module and pam_userpass modules are now unused:

1) On all supported systems, PAM provides a far superior way to
   integrate Kerberos support.
2) The pam_userpass module (https://github.com/openwall/pam_userpass)
   (which is a lovely idea) is no longer maintained.

instfiles/pam.d/Makefile.am

@@ -15,16 +15,8 @@ CLEANFILES = xrdp-sesman
 if SESMAN_NOPAM
 PAMFILE =
 else
-if SESMAN_PAMUSERPASS
-PAMFILE =
-else
-if SESMAN_KERBEROS
-PAMFILE =
-else
 PAMFILE = xrdp-sesman
 endif
-endif
-endif
 
 pamddir = $(pamconfdir)