drop V8::AdjustAmountOfExternalAllocatedMemory usage unless it helps perf

[springmeyer]

We give V8 memory hints with V8::AdjustAmountOfExternalAllocatedMemory, but because this function has to be called from the main loop, and because calling it is not always cheap, it may not be worth it. We don't have hard evidence that telling v8 about memory usage outside its heap actually prevents meltdowns/over mem usage, it is just precautionary.

Also, if we remove these calls, then we also don't need to do anything about #119

[springmeyer]

Going to remove this finally. My reasoning is:

• mapnik.Image objects should be created and destroyed quickly so telling v8 about it is likely unneeded overhead except in memory constrained situations. However memory constrained situations are not common for mapnik rendering
• over time I've watched bugs come and go with usage of V8::AdjustAmountOfExternalAllocatedMemory in node and v8 like nodejs/node@ff74931 and Buffer and ArrayBuffer-based objects trigger mark-sweeps instead of scavenges nodejs/node#1671 and armv6 performance regression nodejs/node#1188 (comment) and indutny/io.js@5bd62bc. These indicate that for objects in core using V8::AdjustAmountOfExternalAllocatedMemory can be critical for keeping memory low. But also that its hard to get this right. Again, because mapnik images should be created and destroyed pretty quickly I don't think it is worth doing this anymore.
• If we do find a benchmark/testcase to demonstrate it helps and we choose to re-instate V8::AdjustAmountOfExternalAllocatedMemory usage then we should also do it for mapnik.VectorTile...