Clean up the mess I created when I cleaned up NemoStuff.jl

[joschmitt]

In #1769, I removed mulmod(::T, ::T, ::Vector{T}) where T <: MPolyRingElem; apparently this is used in Hecke though. (I was certain I ran all the tests locally, but apparently I missed the "long" ones?)

I don't like the name of this function (mod in the context of several multivariate polynomials sounds like Gröbner stuff to me?) and in the end writing divrem(a * b, m)[2] is not much longer.
So, I would only want to introduce this as a deprecation. Is that fine?

Sorry for this.

NEW: I put mulmod in Deprecations.jl but only as a redirect. I also removed another deprecation introduced in #1769 which is actually used in Hecke.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.03%. Comparing base (8d1047a) to head (2d77278).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Deprecations.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1780      +/-   ##
==========================================
- Coverage   88.04%   88.03%   -0.01%     
==========================================
  Files         117      117              
  Lines       30092    30094       +2     
==========================================
  Hits        26494    26494              
- Misses       3598     3600       +2     

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[joschmitt]

The Hecke long tests (with GAP and Polymake and depwarn=error) passed locally, so I hope that this is fine now.