remove dead action_buttons module (superseded by action_bar in Phase L)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

tests/test_drag_out.cpp

@@ -1,5 +1,5 @@
 // Standalone tests for reasampler::drag_out — no REAPER, no test framework. Same fast loop
-// as the sibling pure tests (action_buttons / mode_switch et al.): assert the gesture-
+// as the sibling pure tests (mode_switch et al.): assert the gesture-
 // boundary decision and the path-list assembly directly.
 //
 // Covers (M11 drag-out brief §test cases):