Remove <lib> directives from Solr

<lib> directives in solrconfig.xml used to be recommended way for including additional jar files to the classpath for a particular collection or collections.

For context: This feature required complex handling of "trusted" vs "non-trusted" configsets in configset upload API to keep Solr secure (i.e. to stop RCE attacks for non-authentication enabled deployments). This security feature also broke down recently due to a bug in Schema designer (SOLR-16777).

Supported alternatives exist that are safer:

• user can add the jar files to Solr's classpath
• use packages to use custom jars per collection

In the light of these, there's no need to continue to support the <lib> directive going forward.

I propose to remove the <lib> directives handling and functionality through this issue.