Fix minimal_generating_set for ideals created by a f4 groebner basis

[lgoettgens]

Resolves #4949.

There used to be a singular_assure in the line before the one I edited, but that was removed by @ederc in #2829.

After looking into it for a moment longer, it seems that this line isn't needed at all, as the flag will be set by the singular_generators call in the next line anyway, see

Oscar.jl/src/Rings/mpoly.jl

Lines 389 to 391 in feee19f

	if isdefined(B, :ord) && B.ord == monorder
	S.isGB = B.isGB
	end

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.87%. Comparing base (7f923f6) to head (60864b2).
Report is 17 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4950   +/-   ##
=======================================
  Coverage   84.87%   84.87%           
=======================================
  Files         697      697           
  Lines       93925    93925           
=======================================
+ Hits        79721    79722    +1     
+ Misses      14204    14203    -1     

Files with missing lines	Coverage Δ
src/Rings/mpoly-graded.jl	93.35% <ø> (+0.45%)	⬆️

... and 3 files with indirect coverage changes

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