[1.0 release prep] Remove tilde expansion discipline

Defining a .sh.tilde.get or .sh.tilde.set discipline function to
extend tilde expansion works well as long as the discipline
function doesn't get interrupted (e.g. with Crtl+C) or produce an
error message. Either of those will cause the shell to become
unstable and crash.

This feature is now removed from the 1.0 branch as it is not ready
for prime time. It can return to a release branch if/when we manage
to fix it on the master branch.

Related: https://github.com/ksh93/ksh/issues/346

src/cmd/ksh93/tests/variables.sh

@@ -1026,7 +1026,6 @@ set -- \
 	".sh.math" \
 	".sh.pool" \
 	".sh.pid" \
-	".sh.tilde" \
 	"SHLVL" \
 	"CSWIDTH"