runtime: remove Cond

I don't think this is used anywhere right now, and it would need to be
updated to work with multithreading. So instead of fixing it, I think we
can remove it.

My original intention was to have something like this that could be used
in the machine package, but since this is in the runtime package (and
the runtime package imports the machine package on baremetal) it can't
actually be used that way.

I checked the TinyGo repo and the drivers repo, and `runtime.Cond` isn't
used anywhere except in that one test.

testdata/goroutines.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"runtime"
 	"sync"
 	"time"
 )
@@ -83,8 +82,6 @@ func main() {
 
 	testGoOnInterface(Foo(0))
 
-	testCond()
-
 	testIssue1790()
 
 	done := make(chan int)
@@ -172,47 +169,6 @@ func testGoOnBuiltins() {
 	}
 }
 
-func testCond() {
-	var cond runtime.Cond
-	go func() {
-		// Wait for the caller to wait on the cond.
-		time.Sleep(time.Millisecond)
-
-		// Notify the caller.
-		ok := cond.Notify()
-		if !ok {
-			panic("notification not sent")
-		}
-
-		// This notification will be buffered inside the cond.
-		ok = cond.Notify()
-		if !ok {
-			panic("notification not queued")
-		}
-
-		// This notification should fail, since there is already one buffered.
-		ok = cond.Notify()
-		if ok {
-			panic("notification double-sent")
-		}
-	}()
-
-	// Verify that the cond has no pending notifications.
-	ok := cond.Poll()
-	if ok {
-		panic("unexpected early notification")
-	}
-
-	// Wait for the goroutine spawned earlier to send a notification.
-	cond.Wait()
-
-	// The goroutine should have also queued a notification in the cond.
-	ok = cond.Poll()
-	if !ok {
-		panic("missing queued notification")
-	}
-}
-
 var once sync.Once
 
 func testGoOnInterface(f Itf) {