remove dead action_buttons module (superseded by action_bar in Phase L)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

@@ -1,12 +1,12 @@
 // Standalone tests for reasampler::action_bar — no REAPER, no test framework. Same fast loop
-// as the sibling pure tests (action_buttons / mode_switch / prune_button): assert the
+// as the sibling pure tests (mode_switch / prune_button): assert the
 // task-grouped action-bar layout, overflow-on-narrow, and hit-testing directly.
 //
 // Covers (L2 brief §test cases, updated for L6 single-row face change):
 //   * Layout: correct rects for each action button across representative panel widths; buttons
 //     pack at a fixed width with intra-cluster + inter-cluster gaps.
 //   * Overflow/hiding when the bar is too narrow (whole trailing buttons dropped, never
-//     clipped; earlier frequent clusters survive; mirrors action_buttons suppression).
+//     clipped; earlier frequent clusters survive).
 //   * Label sub-rect correct — spans full button height (L6: keybinding sub-row removed from
 //     the face; binding is in the hover tooltip instead).
 //   * Task grouping reflected STRUCTURALLY: each slot carries its cluster; the flat index runs