Remove FixedSizeArray

[Amanieu]

Closes #27778

[rust-highfive]

r? @kennytm

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[rust-log-analyzer]

The job x86_64-gnu-llvm-10 failed! Check out the build log: (web) (plain)

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   |
15 | use crate::marker::Unsize;
   |     ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`
error: aborting due to previous error

error: could not compile `core`

[klensy]

Already here #84094