[btf] Cleanup AddModulesToSpec

[tpapagian]

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Now (as of 0.14.0) cilium/ebpf handles kmod BTFs natively (https://pkg.go.dev/github.com/cilium/ebpf#ProgramOptions.KernelModuleTypes).

Thus we can remove some duplicate code (i.e. mainly function AddModulesToSpec). This patch does this cleanup.

In Tetragon, we used kmod BTF in 2 places:

1. In order to attach to a function which is now handled automatically by cilium/ebpf.
2. In order to validate if the function name exists in the BTF and to check the argument types. In that case, we need to manually load the spec from the kmod that we care about. This is because cilium/ebpf checks (and loads the appropriate BTF) during the attach phase (i.e. https://github.com/cilium/ebpf/blob/5976561b28aabf23df00f3507cc3b240305b531b/prog.go#L157-L177).

Cleanup code related to handling kmod BTFs.

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