Remove renderKatex feature flag, and simplify code

[gnprice]

We've had KaTeX supported in the last several releases, with the renderKatex experimental feature flag defaulting to true; with #1720 we're doing the same thing in main. We don't plan to ever turn the flag back off.

It's therefore time we should remove the flag and make the simplifications that that enables, in particular cutting out the code that was conditioned on the flag being false. See the doc on [GlobalSettingType.experimentalFeatureFlag]:

  /// Describes a setting which enables an in-progress feature of the app.
  ///
  /// Sometimes when building a complex feature it's useful to merge PRs that
  /// make partial progress, and then to have the feature's logic gated behind
  /// a setting that serves as a "feature flag".
  /// This enables those working on the feature to enable the flag in order to
  /// see the current incomplete behavior, while for everyone else it remains
  /// disabled and so (barring bugs in the use of the flag itself) has no effect.
  ///
  /// These settings are primarily meant for people developing Zulip to use,
  /// and so appear in an out-of-the-way part of the settings UI.
  ///
  /// Settings of this kind are costly to the health of the codebase if
  /// allowed to accumulate.  Most features don't need one, even features that
  /// take two or three PRs to implement.  See discussion at:
  ///   https://github.com/zulip/zulip-flutter/issues/1409#issuecomment-2725793787
  /// When a feature flag is introduced, take care to drive the project to
  /// completion, either by merge or removal, so that the flag can be retired
  /// within a period of a few weeks or months.
  experimentalFeatureFlag,