[Impeller] remove Vulkan pipeline cache mutex. (#43085)

At least from what I've found online, this API is already safe to call from multiple threads. From testing, at startup the presence of the mutex is adding ~100ms to shader creation time, as all compilations across N threads are fighting for the same lock.

https://github.com/flutter/flutter/issues/129050
https://dart.googlesource.com/external/github.com/flutter/engine/+/1225ac5dd6e986b1fe1fecce91408d3eb6db2ed7