nusb/kb: remove mouse acceleration -a flag

mouse acceleration can be done by software in the kernels
devmouse device like:

echo hwaccel off >/dev/mousectl
echo accelerated 5 >/dev/mousectl

it is hard to pass nusb/kb parameters anyway as it is started
from nusbrc in bootfs.paq, so i remove the option.

sys/man/4/nusb

@@ -32,10 +32,7 @@ usbd - Universal Serial Bus drivers
 .B nusb/kb
 [
 .B -d
-] [
-.B -a
-.I n
-] 
+]
 .I devid
 .PP
 .B nusb/serial
@@ -117,12 +114,6 @@ process them.
 Mouse events are sent to
 .BR /dev/mousein
 in the same way.
-.PP
-The option
-.B \-a
-accelerates the mouse to level
-.I n
-(similar to the kernel's mouse driver acceleration).
 .SS Disks
 .I Disk
 configures and manages USB mass storage devices. It

sys/src/cmd/nusb/kb/kb.c

@@ -779,7 +779,7 @@ Err:
 static void
 usage(void)
 {
-	fprint(2, "usage: %s [-d] [-a n] devid\n", argv0);
+	fprint(2, "usage: %s [-d] devid\n", argv0);
 	threadexits("usage");
 }
 
@@ -792,8 +792,6 @@ threadmain(int argc, char* argv[])
 	Usbdev *ud;
 
 	ARGBEGIN{
-	case 'a':
-		break;
 	case 'd':
 		debug++;
 		break;