GUACAMOLE-1298: Trim down NOTICE files to only those attribution notices that are required.

From https://infra.apache.org/licensing-howto.html#bundle-asf-product:

>
> Modifications to NOTICE
> -----------------------
>
> The NOTICE file is reserved for a certain subset of legally required
> notifications which are not satisfied by either the text of LICENSE or
> the presence of licensing information embedded within the bundled
> dependency. Aside from Apache-licensed dependencies which supply NOTICE
> files of their own, it is uncommon for a dependency to require additions
> to NOTICE.
>
> Copyright notifications which have been relocated, rather than
> removed, from source files must be preserved in NOTICE. However,
> elements such as the copyright notifications embedded within BSD and MIT
> licenses do not need to be duplicated in NOTICE. You can leave those
> notices in their original locations.
>
> It is important to keep NOTICE as brief and simple as possible, as each
> addition places a burden on downstream consumers.
>
> *Do not* add anything to NOTICE which is not legally required.
>

doc/licenses/generate-license-files.sh

@@ -327,8 +327,6 @@ EOF
             echo >> "$OUTPUT_DIR/LICENSE"
             trim_file "$LICENSE_FILE" >> "$OUTPUT_DIR/LICENSE"
         fi
-    else
-        error "No recognizable license file within $LICENSE_INFO_DIR"
     fi
 
     # Copy NOTICE, if provided