HTML clients have Apache 2.0 license URL as default

Relates to #140

It looks like the "html" and "html2" client generators use the Apache 2.0 license URL as default if the OpenAPI document does not specify a license:

swagger-codegen-generators/src/main/java/io/swagger/codegen/v3/generators/html/StaticHtmlCodegen.java

Line 58 in e34685e

additionalProperties.put("licenseUrl", "http://apache.org/licenses/LICENSE-2.0.html");

swagger-codegen-generators/src/main/java/io/swagger/codegen/v3/generators/html/StaticHtml2Codegen.java

Line 71 in e34685e

additionalProperties.put("licenseUrl", "http://apache.org/licenses/LICENSE-2.0.html");

This is problematic because it is not clear in the generated HTML documentation what this license applies to, and maybe the developer who publishes the documentation for their API does not want to provide the documentation under that license.

It would be best if this default license URL was omitted, and license information was only shown in case a license is explicitly specified.