Remove pre-ATT&CK domain

[isaisabel]

With the next release of ATT&CK, the pre-ATT&CK domain is being migrated into new tactics under the enterprise domain. We should remove the pre-ATT&CK matrix, techniques/tactics listings, and any other references to pre-ATT&CK as a domain.

We will also need to update the following pages:

• enterprise-introduction may require some revision since it mentions pre-ATT&CK
• pre-introduction should be removed entirely
• FAQ has link to the pre-introduction under "what is ATT&CK"