Remove redundant decl of 509_STORE_set_flags

a47bc283 accidentally adds another define for X509_STORE_set_flags
It is already defined 5lines prior

CLA: trivial

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3047)