inline clippy_utils::ptr into needless_pass_by_value

[ada4a]

While looking into #15569, I stumbled upon clippy_utils::ptr.

This module was created in #2117, to share the logic from ptr_arg with needless_pass_by_value. But then later, #8409 removed the use of the logic from ptr_arg, which left needless_pass_by_value as the only user of this module.

Still, I wanted to try to add docs to the module, but the two functions looked all too specific, so I thought it'd be better to just move them to needless_pass_by_value, in the hopes that whoever is looking at that lint will hopefully have enough context to understand what those functions are doing.

changelog: none

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[samueltardieu]

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[ada4a]

@rustbot ready