[perf] rustdoc: Perform less work when cleaning middle::ty parenthesized generic args

[fmease]

CC #132697. I presume the perf regression it caused (if real) boils down to query invocation overhead, namely of def_kind & trait_def as we don't seem to be decoding more often from the crate metadata.

I won't try the obvious and reduce the amount of query calls by threading information via params as that would render the code awkward.

So instead I'm simply trying to attack some low-hanging fruits in the vicinity.

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Previously, we would clean_middle_generic_args unconditionally inside clean_middle_generic_args_with_constraints even though we didn't actually use its result for parenthesized generic args (Trait(...) -> ...).

Now, we only call clean_middle_generic_args when necessary. Lastly, I've simplified clean_middle_generic_args_with_constraints.

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r? ghost