Remove the deprecated load_xx functions in v0.9.0 (deprecated since v0.6.0)

[seisman]

In PR #1685, we added a single load_sample_data function to load sample data used in tutorials and examples, and deprecated load_xx functions like load_japan_quakes in v0.6.0. These deprecated functions will be removed in v0.9.0.

Since our next targeted release is v0.9.0, now it's time to remove these deprecated functions. IMHO, we should remove these deprecated functions as early as possible, rather than removing them a few days before we make the v0.9.0 release. In this way, we can have more time (3 months vs a few days) to know whether removing these functions would cause any unexpected problems.

This issue is created to track the progress of removing these functions.

• Remove load_japan_quakes: Remove the deprecated load_japan_quakes function (deprecated since v0.6.0) #2301
• Remove load_sample_bathymetry Remove the deprecated load_sample_bathymetry function (deprecated since v0.6.0) #2305
• Remove load_fractures_compilation Remove the deprecated load_fractures_compilation function (deprecated since v0.6.0) #2303
• Remove load_hotspots Remove the deprecated load_hotspots function (deprecated since v0.6.0) #2309
• Remove load_mars_shape Remove the deprecated load_mars_shape function (deprecated since v0.6.0) #2304
• Remove load_ocean_ridge_points Remove the deprecated load_ocean_ridge_points function (deprecated since v0.6.0)  #2308
• Remove load_usgs_quakes Remove the deprecated load_usgs_quakes function (deprecated since v0.6.0) #2306

I've made the PR #2301 to remove the deprecated load_japan_quakes function. This PR could be used as a reference if anyone wants to work on other deprecated functions.

[willschlitzer]

@michaelgrund Since we're working at the same time, I'm holding off on load_hotspots and assuming you're working on it. Let me know if you want me to add a PR to remove it!

[seisman]

Thank @michaelgrund and @willschlitzer for your quick PRs. I think we need to review/merge these PRs one by one to avoid unnecessary conflicts, so please be patient.

[yvonnefroehlich]

In this context, I feel it would be helpful to also work on issue #1774 and the related PR #1814 by @maxrjones for adding a list or an overview of the available sample datasets 🙂. Currently, users do not know directly from the documentation which sample datasets are available via datasets.load_sample_data and have to run datasets.list_sample_data.