Remove heapdump

Remove heapdump.

I've never used it and it's a native Node module we can get rid of during our Node v10 -> v14 upgrade.

Part of https://gitlab.com/gitterHQ/webapp/-/issues/2735

Todo

• Remove usage from gitter-stream/app.js#L13 -> https://gitlab.com/gitterHQ/gitter-stream/-/merge_requests/8
• Remove usage from gitter-webhooks-handler/config/environment.js#L7 -> https://gitlab.com/gitterHQ/gitter-webhooks-handler/-/merge_requests/52

Used in webapp!2156 (merged) to remove the heapdump dependency