Remove unused RESOURCES_DIR cmake option

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Description

Since 27a4c83 the examples are no longer installed.
Now the RESOURCES_DIR option is still parsed but is fully ignored, so remove it.

Tasks

• Tested on Linux
• Tested on Windows
• Tested on macOS
• Tested on iOS
• Tested on Android

[ChrisThrasher]

Good find! This also implies that we can collapse the directory structure of the resources themselves. We can simply put the resources in the same directory as the C++ code. I might do that in a later PR.