Drop the last reference of SHLIB_EXT

Drop the last reference of SHLIB_EXT

SHLIB_EXT is a variable that exists on OpenSSL build file templates before
version 3.0, for which much of the logic for figuring out file names and
such was moved to the 'platform' set of routines.

But, it seems that one (now useless) reference remained on the VMS specific
descrip.mms.tmpl.  We replace it with an explicit '.EXE', which is the
default extension for shared libraries on VMS.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21007)