Remove obsolete login blocks

Problem/Motivation

• #2503755: Switch from user login block to login menu link and search block in standard profile changed the Standard profile to no longer create a login block in Bartik in favor of the standard login link in the user menu. Interestingly it left around a login block for the Seven theme that is generally useless until you somehow grant access for anonymous users to Seven, for example by making it the default theme.
• Umami has the same login block, most probably copied over
• Minimal actually still has a login block for the default theme (Stark). That is not very useful either, though, because the default front page in Minimal is already the login page, where the login bock is never displayed (see also #2801511: Can't place user login block on user/login when using Page Manager and #2364643: Add a minimal default frontpage for context). Minimal also provides the same login link that Standard has so the login block should be obsolete.

Proposed resolution

Remove all three login blocks from the default configuration! Since they do not hurt in any way, we should not add an update function, but just remove it for new installations. As explained above it is very unlikely that people will miss them, and if they are in fact needed for some sites they can easily be re-configured. It just doesn't make sense to provide them by default.