Remove reactivity chapter

The chapter is completely useless, as it is broken, repeats partially
what is approached in the previous chapter and the rest is untested, and
broken.

The disclaimer at the top of the chapter seems to not be enough to
discourage people from reading it.

the_book/SUMMARY.md

@@ -9,7 +9,7 @@
   - [Using the dsl macro](../examples/simple_menu/README.md)
   - [Using a chirp file](../examples/chirp_menu/README.md)
   - [Defining your own DSL](../examples/custom_dsl/README.md)
-  - [Reactivity](reactivity.md)
+  - [Reactivity]()
 - [Examples](../examples/README.md)
   - [`chirpunk`](../examples/chirpunk/README.md)