Remove compiler support for `clearcontext`, `fix`, `switch` statements
and remove corresponding `CODE_` definitions, as well as `CODE_KEY`,
which was never used.
These four statements have never been documented and have either never
been implemented, or long been unsupported. An earlier change (#16445)
marked `clearcontext` and `fix` as deprecated for v19 onwards, but
deprecation for an undocumented language feature is an unnecessary step,
so have opted to remove altogether in v19 to reduce complexity. This
means that `VERSION_190` is used now only for compile target
deprecations.
Follows: #16445
Test-bot: skip
