Remove `norm(::ArbMatrix)`

[lgoettgens]

This is the only matrix type with a norm function, and this is not even a vector-norm induced matrix norm, so not what most people want.

Let's see if this is used downstream, anyway we should wait for the next minor release.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.46%. Comparing base (b04719f) to head (f52e45e). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1764 +/- ## ========================================== - Coverage 85.48% 85.46% -0.03% ========================================== Files 95 95 Lines 36923 36921 -2 ========================================== - Hits 31565 31554 -11 - Misses 5358 5367 +9 ```

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[fingolfin]

Fine by me. But this comes from Hecke, so I wonder if @fieker or @thofma perhaps need(ed) it for an application?

[lgoettgens]

This is indeed used in Hecke in https://github.com/thofma/Hecke.jl/blob/d5a199e6aba6d90ec80922f22eef2ceb2492b4d6/src/RieSrf/Theta.jl#L79 but I still don't think this is a good name. It would be great to hear something from @thofma @fieker @JHanselman

[thofma]

It is the Frobenius norm and is equal to LinearAlgebra.norm. I don't see the problem with the name.

[lgoettgens]

Alright, may I then suggest instead to widen the signature to at least Acb, Complex, Arb and Real (matrices)? And I would add some proper test then

[thofma]

Sure, that sounds good.