Use JuMP native complex support instead of COI

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.13 :warning:

Comparison is base (44b40b4) 80.35% compared to head (1627c63) 80.22%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #298 +/- ## ========================================== - Coverage 80.35% 80.22% -0.13% ========================================== Files 38 41 +3 Lines 2092 2099 +7 ========================================== + Hits 1681 1684 +3 - Misses 411 415 +4 ``` | [Impacted Files](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/298?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev) | Coverage Δ | | |---|---|---| | [src/sos\_polynomial.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/298?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL3Nvc19wb2x5bm9taWFsLmps) | `85.10% <0.00%> (ø)` | | | [src/utilities.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/298?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL3V0aWxpdGllcy5qbA==) | `60.00% <ø> (ø)` | | ... and [9 files with indirect coverage changes](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/298/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev)

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