remove dead action_buttons module (superseded by action_bar in Phase L)

Deletes src/action_buttons.{h,cpp} and tests/test_action_buttons.cpp;
removes the static library, test target, and reaper_reasampler link
entry from CMakeLists.txt; scrubs all dangling comment references.

src/drag_out.h

@@ -24,7 +24,7 @@
 //      drag_out_win — never by relocating or copying bytes here).
 //
 // PURE MODULE: NO REAPER types, NO SWELL, NO OS/OLE, NO vendor/ includes. Standard library
-// only. Builds and unit-tests without REAPER. Mirror of action_buttons / mode_switch.
+// only. Builds and unit-tests without REAPER. Mirror of mode_switch.
 
 #include <string>
 #include <vector>