What do you call that guitar technique where you strum but don't sound the note, but instead make the little "chk chk" or "chf chf" sound

Ghost notes?

FWIW,
ghost notes" usually refers to a single note or two in the middle of a melodic line. The repeated strumming patterns often go by other names. See in particular "palm mute".

I want to clarify some possible misinformation.

palm muting: the intended note is still clear
Popular example: "Every Breath You Take" by The Police
Palm muting can be heard throughout practically the whole song.
The left hand frets normally; notes are picked while the edge of the picking hand mutes the strings.
Sheet music would indicate normal notes with either a + or a . "palm mute" or some combination.

string mute: no defined note
Example: every song you are talking about
Here the fretting hand relaxes pressure so no clear note is heard; the picking hand picks normally.
Sheet music would have an X or a stack of Xs instead of a note head(s).

These really are two different techniques and sounds.