Remove vendored go-securesystemslib

[adityasaky]

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go-securesystemslib is vendored in #264. We must remove this copy of the library once our changes are upstreamed or our requirements are met through other APIs. Note that an effort has started in the in-toto community to make go-securesystemslib more robust, which is the reason we haven't already upstreamed our changes.

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[adityasaky]

FWIW, I'm a maintainer of go-securesystemslib as well, so I'm going to be involved in the go-securesystemslib improvement effort. I will take this issue on when the library is ready for direct use in gittuf.