chore: Remove obsolete runtime/build time configuration setup

[bitsandfoxes]

Description

The changes in #1888 replaced the runtime/build time configuration setup into one Option Configuration setup. The same callback would get invoked wether it is on runtime or at build time.

We kept the old setup around as to not break people when upgrading but it's been 2 mayor releases now.

A migration guide is already in place: https://docs.sentry.io/platforms/unity/migration/#changes-to-the-programmatic-configuration

[linear]

UNITY-166 chore: Remove obsolete runtime/build time configuration setup

[bitsandfoxes]

Upsi. Should have been this PR's description.