http2: remove unused nghttp2 error list

Remove a list of HTTP2 errors as well as `nghttp2_errname()`
that converted an integer nghttp2 error code to a string
representation.

We already use `nghttp2_strerror()` for this, which
is provided by nghttp2 returns a better error string anyway.

PR-URL: https://github.com/nodejs/node/pull/21827


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>