chore: remove zig-server bench binary; don't gitignore artifacts

Drop the accidentally-tracked-in-working-tree zig-server bench binary
(built by bench/run.sh). Reverted the wasm_check* gitignore entry from
bce53b7 — these are removed, not ignored.

.gitignore

@@ -4,6 +4,4 @@ zig-out
 .vscode/
 .sx-cache
 .sx-tmp
-current/
-# emscripten/wasm smoke-test build output
-wasm_check*
+current/