[Obsolescence] Remove holography code

• The holography implementation is removed. It was partly adapted from HyperSpy under GPL v3 and would prevent relicensing of LiberTEM to MIT (#1649). Since it had only few users, development had moved to https://github.com/LiberTEM/LiberTEM-holo, and relicensing to MIT should not be delayed, it is removed without deprecation period and will throw an error on import explaining the situation. The new holography code will hopefully be released in due time. We recommend that users who depend on the previous code use an older LiberTEM version (<= 0.14.1) for the time being and migrate to the new version when appropriate. (#1689)