skin: Remove the id "anonlogin" and merge pt-anonlogin with pt-login

skin: Remove the id "anonlogin" and merge pt-anonlogin with pt-login

This was used as an alternative id for the login link in the
personal tools at the top right for ShowIPinHeader context.

This seems to have been inherited since the days of the
PHPTal-based skins (at least git-blame shows their presence back
in Brion's rewrite of the skin in november 2004).

I've asked several maintainers about this and nobody could figure
out why it would need a different id. After all, whenever the
user sees the login link it means they are using the wiki as an
anonymous user.

My best guess is that this was meant to allow the ability to
style the login differently when the (anonymous) user has
already made an edit (or if we init a session for other reasons).

This is also what toggles the anon userlinks are shown (i.e. the
links to the IP userpage and talkpage), but I see no evidence of
this, and in any case it would be better to change the class of
the whole personal tools div instead of changing the IDs.

Change-Id: Ib3445934853db5361c3c6fecfe75623b0b921400