Remove unused function from es31fSRGBDecodeTests.cpp
Remove unused function from es31fSRGBDecodeTests.cpp

Unused function causes compiler warnings to appear with recent GCC
versions.

Change-Id: I941c579994acbc9146df86acf5b9da9ee85341f5
(cherry picked from commit 1ee59ff98641dc0be7511e03e37b5843ff7eed82)

googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS