Resolve JET warnings / errors

[fingolfin]

Clearly those functions are untested, would be great if they had tests...

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 18.18182% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 85.08%. Comparing base (d619886) to head (5a0cdd2).

:exclamation: Current head 5a0cdd2 differs from pull request most recent head 57e785e. Consider uploading reports for the commit 57e785e to get more accurate results

Files	Patch %	Lines
src/Nemo.jl	0.00%	3 Missing :warning:
src/HeckeMoreStuff.jl	0.00%	2 Missing :warning:
src/flint/nmod_poly.jl	33.33%	2 Missing :warning:
src/flint/fmpz_laurent_series.jl	0.00%	1 Missing :warning:
src/flint/nmod_mat.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1755 +/- ## ========================================== - Coverage 85.46% 85.08% -0.38% ========================================== Files 95 95 Lines 36922 36917 -5 ========================================== - Hits 31554 31411 -143 - Misses 5368 5506 +138 ```

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[thofma]

hm, where did that

function Base.setprecision(a::Generic.MatSpaceElem{QadicFieldElem}, N::Int)
  b = deepcopy(a)
  setprecision!(b, N)
  return b
end

move to @joschmitt?

[joschmitt]

hm, where did that

function Base.setprecision(a::Generic.MatSpaceElem{QadicFieldElem}, N::Int)
  b = deepcopy(a)
  setprecision!(b, N)
  return b
end

move to @joschmitt?

I removed this function because it was nowhere used and broken.