Once bitcode support has been removed from the engine #107884 and recipes #107886, remove bitcode logic from buildroot.

https://github.com/flutter/buildroot/blob/fc82ca44f1ae85c5fbfa06bd313477539ca50729/build/toolchain/clang.gni#L22

Starting with Xcode 14, bitcode is no longer required for watchOS and tvOS applications, and the App Store no longer accepts bitcode submissions from Xcode 14.

See #15288 and flutter/buildroot#272 when bitcode was introduced.