ci: remove libz-dev from macOS build deps

Remove no-longer used libz-dev (from DMG compression).
Followup to #28432.
Missed this in #28932.

depends/README.md

@@ -48,7 +48,7 @@ The paths are automatically configured and no other options are needed unless ta
 
 #### For macOS cross compilation
 
-    sudo apt-get install curl bsdmainutils cmake libz-dev zip
+    sudo apt-get install curl bsdmainutils cmake zip
 
 Note: You must obtain the macOS SDK before proceeding with a cross-compile.
 Under the depends directory, create a subdirectory named `SDKs`.