Remove unnecessary pure annotations

[Keno]

These should now be unnecessary. Our effect modeling of Core.sizeof
has improved sufficiently, that it should be able to know right away
that these functions are pure without the additional annotation.

[Sacha0]

Nifty! :)

[Keno]

These were added by @JeffBezanson in 0b80774, which was #32437 where I now realize I already said that this shouldn't be @pure. At least I'm consistent :)

[KristofferC]

Nice to get rid of the last correct usages of @pure (according to the docs).

[vtjnash]

We explicitly block constant-prop of String, which I think is what made this necessary

[Keno]

Ok, yes, I see (after this PR):

julia> function bar()
       return Base.sizeof("foo")
       end
bar (generic function with 1 method)

julia> bar()
3

julia> @code_typed bar()
CodeInfo(
1 ─ %1 = Core.sizeof::typeof(Core.sizeof)
│   %2 = (%1)("foo")::Int64
└──      return %2
) => Int64

Fortunately, I'm about to fix that too, so we'll just have to hold this PR for a little bit.

[Keno]

about to fix that too

The "about to fix that" is #43852, so this can go in once that's merged.