chore: remove unused code from image optimizer (#98060)
This PR removes some unused code from the image optimizer.
- Remove redundant MIME validation
- Remove unreachable fallback error
These used to be necessary until PR
https://github.com/vercel/next.js/pull/82118 removed the fallback.
Before 82118:
```js
upstreamType = detectContentType(upstreamBuffer) || imageUpstream.contentType?.toLowerCase().trim()
```
After 82118:
```js
upstreamType = detectContentType(upstreamBuffer)
```
Now there is never a case where upstreamType is invalid since its
already been validated, there is no more fallback.
