Kill the syntax `alias this = sym`;

In the pull https://github.com/D-Programming-Language/d-programming-language.org/pull/200, we discussed to replace the syntax `alias this = sym;`, which introduced by the pull #1187.

To keep the discussion continuable, we should not contain the syntax in 2.061 release.
(As far as I know, it is not deeply discussed in @1187.)

@WalterBright , please merge this in 2.061 release.