Use default location for .adoc source in Gradle-based sample

Previously both samples supported Maven and Gradle. Due to the different
default locations between the two build systems, the Gradle-based sample
was configured to use Maven’s default location for its .adoc files.

The samples have now been updated so that one sample uses Maven and the
other uses Gradle. However, when this change was made, the customization
of the Gradle-based sample’s .adoc file location wasn’t removed.

This commit updates the Gradle-based sample to use the default location,
src/docs/asciidoc, for its .adoc source files.

Closes gh-145

samples/rest-notes-spring-hateoas/build.gradle

@@ -48,7 +48,6 @@ test {
 }
 
 asciidoctor {
-	sourceDir 'src/main/asciidoc' // Align with Maven's default location
 	attributes 'snippets': snippetsDir
 	inputs.dir snippetsDir
 	dependsOn test