Remove processing of unused plugin element attributes in feature.xml and distinction between Plug-ins and Fragments in ProductFile

[HannesWell]

This PR consists of two parts:

~~1. Remove unused distinction between Plug-ins and Fragments in ProductFile There is no usage of a product's fragments in P2 or PDE. Consequently PDE support for adding a fragment attribute for fragments listed in a product file can be removed too.~~

Remove processing of unused plugin element attributes in feature.xml Which happens in the context of https://github.com/eclipse-pde/eclipse.pde/issues/730 From the removed attributes only the unpack attribute has been considered in in the FeaturesAction and the RemoteFeaturesAction in order to create corresponding BundleShapeAdvice. But at least for Tycho those advies did not have any effect.

I assume, but I haven't verified it, that BundleShapeAdvices might have an effect in PDE-build tasks. But since a bundle itself should specify its bundle-shape in its MANIFEST.MF I'm fine with removing that support from PDE with this. Nevertheless it requires some rework of the PDE-builds task since they use FeatureEntry.isUnpack()/unpackSet()/..., therefore this is currently a draft.

[github-actions[bot]]

Test Results

       6 files   -     3         6 suites   - 3   34m 12s :stopwatch: + 1m 45s 2 065 tests  - 116  2 063 :heavy_check_mark:  - 114  1 :zzz:  - 3  1 :x: +1  6 285 runs   - 348  6 280 :heavy_check_mark:  - 342  2 :zzz:  - 9  3 :x: +3 

For more details on these failures, see this check.

Results for commit 1d56fea9. ± Comparison against base commit d78c7677.

This pull request removes 116 tests.

``` org.eclipse.equinox.p2.tests.importexport.ImportExportRemoteTests ‑ Unknown test org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testAllowExportFeaturesInstalledFromLocal org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testExportFeaturesInstalledFromLocal org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testIncompatibleP2f org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testLoadP2f org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testLoadUnknownP2f org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testElements org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testEmpty org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testIUs org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testInvalid … ```

:recycle: This comment has been updated with latest results.

[laeubi]

@HannesWell is this still relevant? It requires a manual rebase now!