ast-ksh (ksh2020): REMOVE; superseded by wip/ksh93.

Removing wip/ast-ksh:

- no further development is planned, due to internal collision between ksh
  developers [1] [2]

- it contains unresolved critical bugs [1] [3]

- it derives from ksh93v- branch, at some point developed by Red Hat,
  which is not granted to be backward compatible with the last stable
  ksh93u+ version from AT&T, as ksh93u+m (wip/ksh93) is.

- some features providing by this branch have been already backported to
  the verison provided by wip/ksh93

- the present package looks stale.

- it requires CMake and meson, unlike other ksh93 branches which use
  AST's own build framework and test suite.

  [1] att/ast#1449
  [2] att/ast#1466
  [3] https://groups.google.com/g/korn-shell/c/2VK8kS0_VhA