Set hidden mem witholding to 0 by default

TF doesn't reserve all available memory now. If any
client wants to reserve they can explicitly set
HIP_HIDDEN_FREE_MEM env var
Change-Id: Ied3a948b79f49aa7327f6a820e9789e39cec143b

rocclr/device/device.cpp

@@ -483,10 +483,6 @@ Settings::Settings() : value_(0) {
       GPU_SINGLE_ALLOC_PERCENT = 100;
     }
 
-    if (flagIsDefault(HIP_HIDDEN_FREE_MEM)) {
-      HIP_HIDDEN_FREE_MEM = 320;
-    }
-
     if (flagIsDefault(GPU_FORCE_BLIT_COPY_SIZE)) {
       GPU_FORCE_BLIT_COPY_SIZE = 1024;
     }