mkerr: remove legacy guards from generated error headers

mkerr: remove legacy guards from generated error headers

In pull request #9333, legacy guards were added to the generated
error headers, but the mkerr.pl script was not adjusted accordingly.
So the legacy guards were removed by subsequent `make update` calls.

Fixing the mkerr.pl script properly was disproportionately complicated
by the fact that adding legacy guards only made sense for files which
already existed in version 1.1.1. To keep things simple, it was decided
to drop the legacy guards from the generated headers entirely.

Fixes #10569

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11541)