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).

src/runtime/gc_stack_portable.go

@@ -1,6 +1,5 @@
-//go:build (gc.conservative || gc.none) && tinygo.wasm
-// +build gc.conservative gc.none
-// +build tinygo.wasm
+//go:build gc.conservative && tinygo.wasm
+// +build gc.conservative,tinygo.wasm
 
 package runtime