remote_storage: fix prefix handling in remote storage & clean up

[jcsp]

Problem

Split off from #7399, which is the first piece of code that does a WithDelimiter object listing using a prefix that isn't a full directory name.

Summary of changes

• Revise list function to not append a / to the prefix -- prefixes don't have to end with a slash.
• Fix local_fs implementation of list to not assume that WithDelimiter case will always use a directory as a prerfix.
• Remove list_files, list_prefixes wrappers, as they add little value and obscure the underlying list function -- we need callers to understand the semantics of what they're really calling (listobjectsv2)

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2766 tests run: 2645 passed, 0 failed, 121 skipped (full report)

---

Code coverage* (full report)

• functions: 28.1% (6467 of 23021 functions)
• lines: 46.9% (45895 of 97805 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
1746009 at 2024-04-23T14:19:35.778Z :recycle:}

[VladLazar]

Nice cleanup! Looks good to me, although I'm not super familiar with this code area