Reverted internal prefix for every backend except Dafny (#5538)

### Description
Some clients using Smithy-Dafny were stuck because we changed the
internal field prefix. We revert it in this PR by convenience for them
as we don't have an immediate need for this enhanced prefix.

### How has this been tested?
All tests should pass.

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Co-authored-by: Robin Salkeld <[email protected]>

Source/DafnyCore/Backends/SinglePassCodeGenerator/SinglePassCodeGenerator.cs

@@ -27,9 +27,8 @@ static class SinglePassCompilerExtensions {
  public abstract partial class SinglePassCodeGenerator {
  // Dafny names cannot start with "_". Hence, if an internal Dafny name is problematic in the target language,
  // we can prefix it with "_".
- // However, for backends such as Rust which need special internal fields, we want to clearly
- // disambiguate between compiler-generated names and user-defined names, hence this prefix.
- protected virtual string InternalFieldPrefix => "_i_";
+ // This prefix can be overridden as necessary by backends.
+ protected virtual string InternalFieldPrefix => "_";
  public DafnyOptions Options { get; }
 
  /// <summary>