Configurations/unix-Makefile.tmpl: remove assignment of AS and ASFLAGS

We have never used these variables with the Unix Makefile, and there's
no reason for us to change this, so to avoid confusion, we remove them.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5545)

(cherry picked from commit 61ab6919)