GPUUploadManagerImpl::Page::Writer: remove move-assignment operator #4123

Job	Run time
Linux -> Pre-checks	19s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 4s
Linux -> GCC-x64, Release	10m 34s
Linux -> GCC-x64, Debug	11m 1s
Linux -> Clang-x64, Release	12m 37s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 23s
Linux -> Clang-x64, Debug	13m 6s
	1h 2m 4s

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