Remove IGnosisSafe interface copy

[mdehoog]

This breaks integration into https://github.com/ethereum-optimism/superchain-ops because they depend on the optimism version, and so we get errors:

Error (9553): Invalid type for argument in function call. Invalid implicit conversion from contract IGnosisSafe to contract IGnosisSafe requested.
  --> script/NestedSignFromJson.s.sol:19:14:
   |
19 |         sign(_signerSafe);
   |              ^^^^^^^^^^^

[cb-heimdall]

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