CURLOPT_ALTSVC_CTRL.3: remove CURLALTSVC_ALTUSED

Follow-up to 98c3f14 that removed it from the header file

docs/libcurl/opts/CURLOPT_ALTSVC_CTRL.3

@@ -28,7 +28,6 @@ CURLOPT_ALTSVC_CTRL \- control alt-svc behavior
 #include <curl/curl.h>
 
 #define CURLALTSVC_IMMEDIATELY  (1<<0)
-#define CURLALTSVC_ALTUSED      (1<<1)
 #define CURLALTSVC_READONLYFILE (1<<2)
 #define CURLALTSVC_H1           (1<<3)
 #define CURLALTSVC_H2           (1<<4)
@@ -53,10 +52,8 @@ sure both the source and the destination are legitimate.
 Setting any bit will enable the alt-svc engine.
 .IP "CURLALTSVC_IMMEDIATELY"
 If an Alt-Svc: header is received, this instructs libcurl to switch to one of
-those alternatives asap rather than to save it and use for the next request.
-.IP "CURLALTSVC_ALTUSED"
-Issue the Alt-Used: header in all requests that have been redirected by
-alt-svc.
+those alternatives asap rather than to save it and use for the next
+request. (Not currently supported).
 .IP "CURLALTSVC_READONLYFILE"
 Do not write the alt-svc cache back to the file specified with
 \fICURLOPT_ALTSVC(3)\fP even if it gets updated. By default a file specified