Armature: remove merge function, use dissolve instead

This was crashing, when looking into a fix I noticed that it gave
hap-hazard results dissolving past forks in the parent/child hierarchy
arbitrarily following one chain.

This functionality is almost identical to "dissolve" which delimits
forks in the chain predictably.

So remove this in favor of dissolve (available from the delete menu).

release/scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -4575,7 +4575,6 @@ def km_armature(params):
         ("armature.extrude_forked", {"type": 'E', "value": 'PRESS', "shift": True}, None),
         ("armature.click_extrude", {"type": params.action_mouse, "value": 'CLICK', "ctrl": True}, None),
         ("armature.fill", {"type": 'F', "value": 'PRESS'}, None),
-        ("armature.merge", {"type": 'M', "value": 'PRESS', "alt": True}, None),
         ("armature.split", {"type": 'Y', "value": 'PRESS'}, None),
         ("armature.separate", {"type": 'P', "value": 'PRESS'}, None),
         # Set flags.

release/scripts/startup/bl_ui/space_view3d.py

@@ -4636,7 +4636,6 @@ class VIEW3D_MT_edit_armature(Menu):
             layout.operator("armature.extrude_forked")
 
         layout.operator("armature.duplicate_move")
-        layout.operator("armature.merge")
         layout.operator("armature.fill")
         layout.operator("armature.split")
         layout.operator("armature.separate")
@@ -4710,7 +4709,6 @@ class VIEW3D_MT_armature_context_menu(Menu):
         # Remove
         layout.operator("armature.split")
         layout.operator("armature.separate")
-        layout.operator("armature.merge")
         layout.operator("armature.dissolve")
         layout.operator("armature.delete")