Properly free the underlying struct.
Properly free the underlying struct.

The weak callback on the proxy object was unnecessary, and added extra
complexity. Now there's no more memory leak! Thanks @indutny!

KenanSulayman/node-weak,fastest963/node-weak,KenanSulayman/node-weak,fastest963/node-weak,unbornchikken/node-weak,TooTallNate/node-weak,kkoopa/node-weak,robcolburn/node-weak,EvolveLabs/electron-weak,robcolburn/node-weak,EvolveLabs/electron-weak,unbornchikken/node-weak,robcolburn/node-weak,TooTallNate/node-weak,kkoopa/node-weak,KenanSulayman/node-weak,TooTallNate/node-weak,kkoopa/node-weak,fastest963/node-weak,EvolveLabs/electron-weak,unbornchikken/node-weak