[vm] Remove incorrect assert in Script::GetSnippet

In Dart 2, sources could be unavailable in other kinds of snapshots
besides FullAOT, so this assertion is not correct.

Fixes dart-lang/sdk#34872

Change-Id: I1b96b353e7a2cdf9c8247cdc7c06b08e3629c78b
Reviewed-on: https://dart-review.googlesource.com/c/81283
Auto-Submit: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

runtime/vm/object.cc

@@ -9858,7 +9858,6 @@ RawString* Script::GetSnippet(intptr_t from_line,
                               intptr_t to_column) const {
   const String& src = String::Handle(Source());
   if (src.IsNull()) {
-    ASSERT(Dart::vm_snapshot_kind() == Snapshot::kFullAOT);
     return Symbols::OptimizedOut().raw();
   }
   intptr_t length = src.Length();