Remove PEAssembly::UrlToPath and PEAssembly::FindLastPathSeparator

[Copilot]

Remove two dead/thin static helpers from PEAssembly:

• UrlToPath — no callers; deleted outright.
• FindLastPathSeparator — was a one-liner wrapping SString::FindBack; inlined at its two call sites in nativeimage.cpp and nativelibrary.cpp.

cc @dotnet/appmodel @AaronRobinsonMSFT

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