Unnecessary `nvtx` dependency

This is likely leftover from when there used to be `nvtx` usage directly in httomo: https://github.com/DiamondLightSource/httomo/blob/f619ed123f2c5aad790ad75751d737c817a44196/conda/environment.yml#L16

However, it looks like `nvtx` is not used anywhere in the httomo codebase anymore (all usages must have gone into the methods repos, which would make sense), so this dependency in httomo can probably be removed.