Drop 'Last Modified' field

[adamchainz]

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.

[manfre]

Do we want the :Last-Modified: to reflect when the content/substance of the DEP has last changed, instead of the last time anything in the file was changed? If the latter, then we should definitely remove them.

[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 👍 from me on this!

[carltongibson]

+1.

[hvdklauw]

Isn't this whole PR a reason to keep the field? If this goes through then all the files histories get an extra commit to change the structure of the files thus polluting the git history with irrelevant history for the specific DEP.

Before you know it you end up going through a long history just to find the last actual change relevant to the subject matter in the DEP.

I think you should keep the meta-data in the file or adopt a standard to differentiate between content commits and structure commits, so it's easier to see the difference.

But that's just my 2 cents.