Remove 'varsize-limit'

This makes the Ada-specific "varsize-limit" a synonym for
"max-value-size", and removes the Ada-specific checks of the limit.

I am not certain of the history here, but it seems to me that this
code is fully obsolete now.  And, removing this makes it possible to
index large Ada arrays without triggering an error.  A new test case
is included to demonstrate this.