Remove unused stitchAsyncCode fishhook code

### Description

Remove all coded added with https://github.com/getsentry/sentry-cocoa/pull/998 and subsequent PRs, as we decided to sunset this feature in https://github.com/getsentry/sentry-cocoa/issues/1786#issuecomment-1542095892 and move forward with https://github.com/getsentry/sentry-cocoa/issues/1919 instead.