gui: remove macOS start on login code

The macOS startup item code was disabled for builds targeting macOS >
10.11 in #15208. Now that we require macOS 10.12 as a minimum, #17550,
we can remove the startup item code entirely, as the API we were using
was removed in macOS 10.12.

doc/release-notes.md

@@ -86,6 +86,8 @@ Low-level Changes section below.
 GUI changes
 -----------
 
+- The "Start Bitcoin Core on system login" option has been removed on macOS.
+
 Wallet
 ------