Drop 'Last Modified' field

[adamchainz]

trafficstars

Following discussion in #62, realized this field is out of sync, we can retrieve the information from Git, and several DEP's never had the field.

[adamchainz]

Even if it's the former, I believe it's out of sync for most of them.

[adamchainz]

@felixxm thoughts?

[RealOrangeOne]

Speaking from my experiences on #86, I often forget to update the Last-Modified field. Sure, it's useful as a marker for "when did this content notably change", but you can get the majority of that from commit history.

Big +1 :+1: from me on this!

[carltongibson]

+1.