Remove deprecated add* APIs

Fixes #2652

typings/xterm-headless.d.ts

@@ -630,18 +630,13 @@ declare module 'xterm-headless' {
     resize(columns: number, rows: number): void;
 
     /**
-     * (EXPERIMENTAL) Adds a marker to the normal buffer and returns it. If the
-     * alt buffer is active, undefined is returned.
+     * Adds a marker to the normal buffer and returns it. If the alt buffer is
+     * active, undefined is returned.
      * @param cursorYOffset The y position offset of the marker from the cursor.
      * @returns The new marker or undefined.
      */
     registerMarker(cursorYOffset?: number): IMarker | undefined;
 
-    /**
-     * @deprecated use `registerMarker` instead.
-     */
-    addMarker(cursorYOffset: number): IMarker | undefined;
-
     /*
       * Disposes of the terminal, detaching it from the DOM and removing any
       * active listeners.

typings/xterm.d.ts

@@ -912,18 +912,13 @@ declare module 'xterm' {
     deregisterCharacterJoiner(joinerId: number): void;
 
     /**
-     * (EXPERIMENTAL) Adds a marker to the normal buffer and returns it. If the
-     * alt buffer is active, undefined is returned.
+     * Adds a marker to the normal buffer and returns it. If the alt buffer is
+     * active, undefined is returned.
      * @param cursorYOffset The y position offset of the marker from the cursor.
      * @returns The new marker or undefined.
      */
     registerMarker(cursorYOffset?: number): IMarker | undefined;
 
-    /**
-     * @deprecated use `registerMarker` instead.
-     */
-    addMarker(cursorYOffset: number): IMarker | undefined;
-
     /**
      * (EXPERIMENTAL) Adds a decoration to the terminal using
      *  @param decorationOptions, which takes a marker and an optional anchor,