Remove ProfilePhase CHECKs.
Remove ProfilePhase CHECKs.

For unit tests, we end up running ProfilePhase constructors/destructors
even though the stats system isn't running.

shihchinw/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3