Build cleanup: don't use SHARED_SOURCE with modules

SHARED_SOURCE is reserved for products that are expected to come in
dual shared / non-shared form, i.e. the routine libraries like
libcrypto and libssl, to distinguish source that should only appear in
their shared form.

Modules are always shared, so there's no need for them to have this
type of distinction.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8623)