Remove .gitignore from the Documentation subfolder

The documentation is now built with buck and asiidoctor, and the
generated html files are output into ./buck-out/gen/Documentation
rather than ./Documentation.

The .gitignore file in ./Documentation was set up to ignore files
that used to be generated by asciidoc, and is no longer needed.

Change-Id: I0311f7ac52ff1f1220c8a8def93b10b9ec7dfc7e
1 file changed