all: remove ineffective _ = variable assignments (#530)

Lastest Zig complains about this, so they must removed to build.

libs/ecs/src/entities.zig

@@ -342,7 +342,6 @@ pub const void_archetype_hash = std.math.maxInt(u64);
 /// * ComponentStorage(T) is a column of data within a table for a single type of component `T`.
 pub fn Entities(comptime all_components: anytype) type {
     // TODO: validate all_components is a namespaced component set in the form we expect
-    _ = all_components;
     return struct {
         allocator: Allocator,
 

libs/glfw/src/Joystick.zig

@@ -62,13 +62,11 @@ const GamepadState = extern struct {
 
     /// Returns the state of the specified gamepad button.
     pub fn getButton(self: @This(), which: GamepadButton) Action {
-        _ = self;
         return @intToEnum(Action, self.buttons[@intCast(u32, @enumToInt(which))]);
     }
 
     /// Returns the status of the specified gamepad axis, in the range -1.0 to 1.0 inclusive.
     pub fn getAxis(self: @This(), which: GamepadAxis) f32 {
-        _ = self;
         return self.axes[@intCast(u32, @enumToInt(which))];
     }
 };

libs/sysaudio/src/soundio.zig

@@ -73,7 +73,6 @@ pub const Device = struct {
                         frame_count_min: c_int,
                         frame_count_max: c_int,
                     ) callconv(.C) void {
-                        _ = frame_count_min;
                         const outstream = SoundIoOutStream{ .handle = @ptrCast(*c.SoundIoOutStream, c_outstream) };
                         const device = @ptrCast(*Device, @alignCast(@alignOf(Device), outstream.handle.userdata));