`InMemoryOnDiskCorpus::with_meta_format_and_prefix` does not actually add a prefix to files

It looks like the `prefix` field of `InMemoryOnDiskCorpus` can be set using `with_meta_format_and_prefix`, but it is actually never used to compose the name of the corpus files [as the documentation states][1]. The same holds for `OnDiskCorpus::with_prefix`, which offloads that functionality to this struct.

https://github.com/AFLplusplus/LibAFL/blob/fb5772e425fef60cfa7529488c98542254b1e543/crates/libafl/src/corpus/inmemory_ondisk.rs#L55-L65

It looks like the `prefix` field was added as part of https://github.com/AFLplusplus/LibAFL/pull/981 but never completely implemented.

[1]: https://docs.rs/libafl/latest/libafl/corpus/inmemory_ondisk/struct.InMemoryOnDiskCorpus.html#method.with_meta_format_and_prefix