Auto merge of #30087 - petrochenkov:indi, r=nrc

I've measured the time/memory consumption before and after - the difference is lost in statistical noise, so it's mostly a code simplification.
Sizes of `enum`s are not affected.

r? @nrc

I wonder if AST/HIR visitors could run faster if `P`s are systematically removed (except for cases where they control `enum` sizes). Theoretically they should.
Remaining unnecessary `P`s can't be easily removed because many folders accept `P<X>`s as arguments, but these folders can be converted to accept `X`s instead without loss of efficiency.
When I have a mood for some mindless refactoring again, I'll probably try to convert the folders, remove remaining `P`s and measure again.

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