Cleanup asciidoctor build

Remove Guardfile for now; we might reintroduce it later, but it is
currently out of sync with the documentation changes.

Do not ignore html files in the asciidoc directory as those are needed
for docinfo parts.

See gh-12611

spring-boot-project/spring-boot-docs/src/main/asciidoc/.gitignore

-*.html
-*.css

spring-boot-project/spring-boot-docs/src/main/asciidoc/Guardfile

-require 'asciidoctor'
-require 'erb'
-
-guard 'shell' do
-	watch(/.*\.adoc$/) {|m|
-		Asciidoctor.render_file('index.adoc', \
-			:in_place => true, \
-			:safe => Asciidoctor::SafeMode::UNSAFE, \
-			:attributes=> { \
-				'source-highlighter' => 'prettify', \
-				'icons' => 'font', \
-				'linkcss'=> 'true', \
-				'copycss' => 'true', \
-				'doctype' => 'book'})
-	}
-end
-
-guard 'livereload' do
-	watch(%r{^.+\.(css|js|html)$})
-end