Remove IMavenProjectFacade#getMavenProject()

[laeubi]

I have think more about IMavenProjectFacade#getMavenProject() now, look for usages in m2e code and how it behaves and came to the conclusion that it is actually bad to have this in the API (and thus like to deprecate/for removal) it.

• From all 37 reference in the codebase only 8 actually checking for null the rest sooner or later de-reference the project and will fail if null is returned.
• From the few actually checking for null, this almost always results in "nothing" happens even though the user would expect something to happen

As whether or not a project is cached is not deterministic, there are only two choices:

1. The project is actually loaded at this point (because from the callstack / context) and it doesn't matter what method is called
2. Whether or not the code do useful things or even runs into a NPE is completely random and depends among other things how many projects are actively used in the workspace

Because of this I think:

1. We should deprecate it from the public API, this method is just confusing and of less use for consumers (but might be of rare use by internal code that carefully is crafted to react fro the consequences)
2. Probably check if we should cache some more data in the facade if it is useful for consumers so they don't need to query the maven project at all
3. Improve general performance / memory footprint of m2e so we can hold more data in the cache and doing caching smarter (e.g as suggested by @mickaelistria here ProjectRegistryManager and caching #1032 (comment)) so even if consumers need the model it is most probably already cached.