Should authors omit statements / citations of well known results in response to referee requests?
I would recommend approaching a referees recommendation to drop a citation in the following manner:


First, consider carefully whether there is good reason to keep a citation in the paper at all.  The key factors to consider are reading audience and the customs of others in the field.  For example, in cross-disciplinary work I have often cited extremely basic material because core dogma known by every undergraduate in one field are sometimes viewed as nearly unbelievable statements in another field.  On the other hand, students in particular are sometimes prone to over-citation because they have been well-trained in citation and decide to err on the side of caution.
If a citation is warranted, consider whether to use the primary source or a textbook source.  Notation and interpretation change over the years, and for purposes of elucidating a well-known result, a readily accessible modern text is often a much better citation for the reader than a difficult to find or understand original.  In certain cases, however, returning to the original is important: for example, the standard interpretation of the Turing test is different than what Turing originally proposed.
Whether the decision is to keep the same citation, change the citation, or drop it all-together, in responding to the referee one should explain carefully why one made that decision.  I would be startled if a paper could be imperiled by deciding to keep a citation when a referee recommended dropping it---it's just such a minor issue, all things considered.  With a careful explanation of how the choice has been reconsidered and the final decision, most referees are likely to be sufficiently satisfied, even if they might have chosen differently.


Bottom line: no paper is going to be put in peril by a few extra citations as long as the author is reasonable in explaining their decisions.  The citations might ultimately get dropped or not, but in every case we are talking about "minor revisions" territory and not the boundary between "major revision" and "reject."