how to \cite without an entry in the bibliography?

You could:

1. Set the skipbib option on the particular key in your .bib file.

2. Define a category with \DeclareBibliographyCategory{dontbib} and then \addtocategory{dontbib}{key} to put individual works into that category, followed by \printbibliography[notcategory=dontbib].

3. Create a cite command to add to that category automatically, if you needed this often. At which point you'd have to decide whether a \cite in the ordinary way should override your exclusion from the bibliography. To do that I think' you'd need two categories, where an ordinary \cite effectively sets one flag, and your special one sets the other, and you set up a bibfilter to adjudicate between them.