RFR: 8259864: Cleanup unused Prism files

[Johan Vos]

On Sat, 16 Jan 2021 21:30:17 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> There are references to `eglx11` and `eglfb` in some of the `buildSrc/*.gradle` files. I recommend cleaning those up, too.
>
> I also see references in `ES2Pipeline.java`, `GLFactory.java` and `PrismES2Defs.h`. Should those be removed, too?

I removed more references.
Note that not all references are removed, since e.g. the flag EGL_FB might also be passed to the c compiler when compiling for non-eglfb systems. This requires another cleanup.

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PR: https://git.openjdk.java.net/jfx/pull/378