Revert "Enable wasm pointer tracking for gc=none."

This reverts commit 0b3a7280fa and updates
the documentation a little bit to explain the purpose of -gc=none. (I'm
thinking about the attiny10 by the way where defaulting to -gc=none
makes sense).

compileopts/config.go

@@ -105,8 +105,6 @@ func (c *Config) GC() string {
 // that can be traced by the garbage collector.
 func (c *Config) NeedsStackObjects() bool {
 	switch c.GC() {
-	case "none":
-		fallthrough
 	case "conservative":
 		for _, tag := range c.BuildTags() {
 			if tag == "tinygo.wasm" {