src: remove defunct timer_wrap file

Unused since the excellent refactoring in 2930bd,
which also removed `src/timer_wrap.cc` from `node.gyp`. If you try and
get at the binding on a v11.0-pre build, you'll get an error, since the
file is no longer in the GYP build.

```
Jonathans-MBP:node jon$ ./node -v
v11.0.0-pre
Jonathans-MBP:node jon$ ./node -e "process.binding('timer_wrap')"
internal/bootstrap/loaders.js:81
        mod = bindingObj[module] = getBinding(module);
                                   ^

Error: No such module: timer_wrap
    at process.binding (internal/bootstrap/loaders.js:81:36)
    at [eval]:1:9
    at Script.runInThisContext (vm.js:89:20)
    at Object.runInThisContext (vm.js:286:38)
    at Object.<anonymous> ([eval]-wrapper:6:22)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at evalScript (internal/bootstrap/node.js:562:27)
    at startup (internal/bootstrap/node.js:248:9)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:595:3)
```

PR-URL: nodejs#21777
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Gus Caplan <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Refs: nodejs#20894