Remove `sptr` dependency once `1.84.0` is released

[CrazyboyQCD]

Since rust-lang/rust#130350 has landed, sptr could be removed after release.

[jedel1043]

Blocked by xd009642/tarpaulin#1642

[xd009642]

There is a workaround for that in LD_LIBRARY_PATH="$(rustc --print=target-libdir)" cargo tarpaulin -o html but also I have a PR branch if you want to test it and see if it resolves the issue xd009642/tarpaulin#1680 (I will find some time to test later today or tomorrow)

[jedel1043]

@xd009642 Thank you for the fix, it works wonders!