prevent wvuthorns/IllinoisGRMHD from compiling in release past 2024_05

[Roland Haas]

With ET_2024_05 there are now 2 thorns called IllinoisGRMHD in the repos directory. Only one of them is in the ET (arrangements/GRHayLET/IllinoisGRMHD).

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To avoid users accidentally using the old thorn in the (familiar) wvuthorns repository it may be a good idea to:

1. add warning to its README file that this thorn is outdated, no longer maintained and superseded by the one in GRHayLET
2. add a “bomb” to the source code that shows that same information. Similar to eg https://github.com/rhaas80/Outflow/commit/506c08d7139a5f4ed8aac4acf8658611fa3c03f5