fix(number_theory/number_field): make ring_of_integers_algebra not an instance (#12331)
fix(number_theory/number_field): make ring_of_integers_algebra not an instance (#12331)

This issue has caused problems for at least two of Kevin's PhD students; it is better to remove it for now or disable it temporarily.

c.f. https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/diamond.20for.20monoid.20instance.20on.20ideals



Co-authored-by: Eric Rodriguez <480064f38bcfd78eaf0148211ef61a38099706aa@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib