Remove redundant declarations of ERR_load_*_strings()

In commit 52df25cf

, the
ERR_load_FOO_strings() functions were moved from their original
location in foo.h into new headers called fooerr.h.  But they were
never removed from their original locations.  This duplication
causes redundant-declaration warnings on programs that use OpenSSL's
headers with such warnings enabled.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5150)