Code cleanup: remove the VMS specific reimplementation of gmtime

This reimplementation was necessary before VMS C V7.1.  Since that's
the minimum version we support in this OpenSSL version, the
reimplementation is no longer needed.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2762)
(cherry picked from commit 9d70ac97)