remove unused code

[QiJune]

Fix #5218
We should set GFLAGS using ParseCommandLineFlags interface

[jacquesqiao]

LGTM!

[wangkuiyi]

I agree that we don't need platform/environment.{h,cc}, which looks too heavy. This segment of code could be

if (std::getenv(kEnvFractionGpuMemoryToUse)) {
  *FLAG_fraction_of_gpu_memory_to_use = std::strtod(std::getenv(kEnvFractionGpuMemoryToUse), nullptr);
  PADDLE_ENFORCE_GT(val, 0.0);		
  PADDLE_ENFORCE_LE(val, 1.0);
}