Skip rewriting Mach-O binaries when delete_rpath changes nothing

[hyuraku]

MachOShim#delete_rpath always wrote the whole binary back to disk, even when there was no matching rpath to remove.
This change makes it return early when there is nothing to delete, so it no longer writes the file for a no-op. It now returns the deleted rpath, or nil when nothing was deleted, which matches how Array#delete and Hash#delete behave.
Keg#delete_rpath turns that value into a boolean, so the relocation step no longer codesigns files that were never modified. This resolves the TODO that asked to delay the write! call until we know we are changing the rpaths.

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AI (Claude Code) assisted in writing the code and tests.
I reviewed the change and ran brew lgtm locally

[MikeMcQuaid]

Great idea/catch!