Remove a non-useful type check on generic type variable (#3457)

* Invert a type test on a type variable

This type test was introduced during the null safety migration and I do
not see discussion about the intention.
In practice this is `assert(true)` so it would be safest to remove it
entirely, but we can also take a guess at the intention.

The full assert when it was introduced was
`asset(T is! Iterable || splitCommas == true)`.

Later the second condition was removed, and the `splitCommas` field
overridden to always return `false`.

I think the intention was to express that options which support multiple
values _must_ split commas, and now that this class is only used without
splitting commas the intention is that the options must be scalar
values.

Invert the type test with a value known to satisfy a check for any
subtype of `Iterable`. Add a string explaining the assert.

* Remove assert entirely

Author: natebosch

lib/src/dartdoc_options.dart

@@ -599,8 +599,7 @@ class DartdocOptionArgSynth<T> extends DartdocOption<T>
       String help = '',
       OptionKind optionIs = OptionKind.other,
       this.negatable = false})
-      : assert(T is! Iterable),
-        super(name, null, help, optionIs, mustExist, null, resourceProvider);
+      : super(name, null, help, optionIs, mustExist, null, resourceProvider);
 
   @override
   bool get hide => false;