Merge bitcoin#2026: [Cleanup] [Trivial] Remove stale siphash.py file

578e308 [Cleanup] Remove stale siphash.py file (Fuzzbawls)

Pull request description:

  This file managed to get reintroduced after we moved the regression test
   suite to the `test` folder. Remove it as it isn't used anywhere.

ACKs for top commit:
  random-zebra:
    Yeah, the one we use is the `test` folder. utACK 578e308
  furszy:
    utACK 578e308

Tree-SHA512: 5a96e8b9086ce6ecea7a5e20416a6021e59c756144f98a34160fa8d1d7da2b0ed97df1ab3fad67d281e02bdcd3cd282dd01190a24ebff14fcac354fae4b58560