Remove deprecated metrics API

### Description

[The JS metric API was deprecated](https://github.com/getsentry/sentry-javascript/pull/14157) with [sentry-javascript 8.37.0](https://github.com/getsentry/sentry-javascript/releases/tag/8.37.0) and needs to be removed ([see usages](https://github.com/search?q=repo%3Agetsentry%2Fsentry-react-native+Sentry.metrics&type=code))