[Cleanup][Kernel] Remove if-else with identical branches in marlin 2:4

[tlrmchlsmth]

Clean up a useless if-else in marlin_24_cuda_kernel.cu:

vllm/csrc/quantization/marlin/sparse/marlin_24_cuda_kernel.cu

Lines 299 to 305 in 0a4d968

	if (group_blocks != -1) {
	s_sh_rd = 8 * ((threadIdx.x / 32) % (thread_n_blocks / 4)) +
	(threadIdx.x % 32) / 4;
	} else {
	s_sh_rd = 8 * ((threadIdx.x / 32) % (thread_n_blocks / 4)) +
	(threadIdx.x % 32) / 4;
	}

See the equivalent if-else in marlin_cuda_kernel.cu:

vllm/csrc/quantization/marlin/dense/marlin_cuda_kernel.cu

Lines 367 to 372 in 80ca1e6

	if (group_blocks != -1)
	s_sh_rd = 8 * ((threadIdx.x / 32) % (thread_n_blocks / 4)) +
	(threadIdx.x % 32) / 4;
	else
	s_sh_rd = 8 * ((threadIdx.x / 32) % (thread_n_blocks / 4)) +
	(threadIdx.x % 32) % 4;

See discussion in #6030

From @alexm-neuralmagic in that issue:

this is a leftover from a copy-paste from the original (dense) marlin that had different branches. We have tests to verify all of these cases inside test_marlin_gemm.py that verify both dense and sparse and group_size == -1 and other group_sizes as well.

I'd love to leave a better explanation for the discrepancy between the equivalent if statement in marlin_cuda_kernel.cu, because this looks very suspicious.

(closes #6030)

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[mgoin]

Thanks for addressing :)