Remove getElement from NativeDgnDb

[6ar8nas]

Removing getElement from NativeDgnDb.

The alternative for it can be now accessed using getInstance together with mapping function on itwinjs-core itwin/core-common package (implemented with iTwin/itwinjs-core#6549)

Closes iTwin/itwinjs-backlog#1153

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ 6ar8nas
❌ Sarunas.Griskus

---

Sarunas.Griskus seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}