Remove scriptExtensions config option

[keeganwitt]

Instead having to make sure your <scriptExtensions> match what you declared in your <sources> when using a non-standard Groovy file extensions to get stubs to work, GMavenPlus should detect the extensions from the declared sources and automatically use those.

[keeganwitt]

Obviously this is a breaking change for any builds using scriptExtensions.

[keeganwitt]

We are also inconsistent without this change because the compile mojo only accepts .groovy files, even though the stubs by default accept groovy, gvy, gy, and gsh files. These are obviously incompatible with each other, so the fact the stub generation is more generous is irrelevant.