[Misc] Remove Gemma RoPE

[WoosukKwon]

This PR removes GemmaRotaryEmbedding because it is actually the same as the normal RotaryEmbedding.

This is because

torch.arange(0, self.rotary_dim, 2, dtype=torch.float)

is equal to

torch.arange(0, self.rotary_dim, 2, dtype=torch.int64).float()

as rotary_dim is less than or equal to head_size (i.e., 256).

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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