Remove "Property based testing" from planned documentation

Triq will be documented in its own plugin later on.

doc/src/guide/book.asciidoc

@@ -46,8 +46,6 @@ include::eunit.asciidoc[EUnit]
 
 include::common_test.asciidoc[Common Test]
 
-include::property_based_testing.asciidoc[Property based testing]
-
 include::coverage.asciidoc[Code coverage]
 
 include::ci.asciidoc[Continuous integration]

doc/src/guide/property_based_testing.asciidoc

@@ -1,6 +0,0 @@
-[[property_based_testing]]
-== Property based testing
-
-// @todo Write it.
-
-Placeholder chapter.