Remove broken support for PEM-encoded SSH public keys

[silverwind]

Trying to add a PEM-encoded public SSH key like

-----BEGIN RSA PUBLIC KEY-----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-----END RSA PUBLIC KEY-----

on a user or repo results in this weird error coming from here:

extractTypeFromBase64Key: invalid key format: not enough length 813826442

This has probably been broken for years.

[zeripath]

OK so there appear to be two different type of keys here - neither of which appear to work.

There is the SSH2 key type which I think is what this code is supposed to work for - but doesn't because they start with ---- BEGIN not -----BEGIN.

And there is the PEM type which is what you've tried here.

[silverwind]

Yeah, it looks like the code is meant for this format:

---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-20160816"
AAAAB3NzaC1yc2EAAAABJQAAAQEApoYJFnGDNis/2oCT6/h9Lzz2y0BVHLv8joXM
s4SYcYUVwBxNzqJsDWbikBn/h32AC36qAW24Bft+suGMtJGS3oSX53qR7ozsXs/D
lCO5FzRxi4JodStiYaz/pPK24WFOb4sLXr758tz2u+ZP2lfDfzn9nLxregZvO9m+
zpToLCWlXrzjZxDesJOcfh/eszU9KUKXfXn6Jsey7ej8TYqB2DgYCfv8jGm+oLVe
UOLEl7fxzjgcDdiLaXbqq7dFoOsHUABBV6kaXyE9LmkbXZB9lQ==
---- END SSH2 PUBLIC KEY ----

The way the fences are written there will not match our check for -----BEGIN which I added assuming it was for above format. I guess since no one complained about these keys not being supported, I guess we may as well opt to drop support for them given that they can easily be converted to the OpenSSH format.

I also just tested on GitHub and they only support the OpenSSH format.

[zeripath]

So #7600 should fix this.