Revert ProductFun space promotion

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.02% :warning:

Comparison is base (cd5585f) 27.58% compared to head (df29f6a) 27.56%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #544 +/- ## ========================================== - Coverage 27.58% 27.56% -0.02% ========================================== Files 80 80 Lines 8374 8380 +6 ========================================== Hits 2310 2310 - Misses 6064 6070 +6 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Multivariate/ProductFun.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL011bHRpdmFyaWF0ZS9Qcm9kdWN0RnVuLmps) | `0.00% <0.00%> (ø)` | | | [src/PDE/KroneckerOperator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1BERS9Lcm9uZWNrZXJPcGVyYXRvci5qbA==) | `3.92% <0.00%> (-0.10%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/544/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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