Remove experimentalCharAtlas, make dynamic default

The NoneCharAtlas is still available to swap in if we need it.

Fixes #1576

typings/xterm.d.ts

@@ -83,24 +83,6 @@ declare module 'xterm' {
      */
     enableBold?: boolean;
 
-    /**
-     * What character atlas implementation to use. The character atlas caches drawn characters,
-     * speeding up rendering significantly. However, it can introduce some minor rendering
-     * artifacts.
-     *
-     * - 'none': Don't use an atlas.
-     * - 'static': Generate an atlas when the terminal starts or is reconfigured. This atlas will
-     *   only contain ASCII characters in 16 colors.
-     * - 'dynamic': Generate an atlas using a LRU cache as characters are requested. Limited to
-     *   ASCII characters (for now), but supports 256 colors. For characters covered by the static
-     *   cache, it's slightly slower in comparison, since there's more overhead involved in
-     *   managing the cache.
-     *
-     * Currently defaults to 'static'. This option may be removed in the future. If it is, passed
-     * parameters will be ignored.
-     */
-    experimentalCharAtlas?: 'none' | 'static' | 'dynamic';
-
     /**
      * The font size used to render text.
      */