gnu: Promote libyaml@0.2 to the default 'libyaml' variable.

* gnu/packages/web.scm (libyaml): Update to 0.2.4.
(libyaml-0.2): Remove variable.
* gnu/packages/haskell-xyz.scm (ghc-libyaml)[inputs]: Change from LIBYAML-0.2
to LIBYAML.