Remove support for systems without ftello/fseeko

fseeko and ftello conforms to SUSv2, POSIX.1-2001. The configure.in says:

# BSDI doesn't have ftello fseeko
AC_CHECK_FUNCS(ftello fseeko)

The last version of BSD/OS was released in 2003, henceforth I believe it's safe to remove this workaround.