ignore help mojo inside m2e workspace

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
Removed 'disappearing maven/plugin.xml' from known problems

It was caused by unrelated and now fixed problem with m2e/modello
integration

https://github.com/tesla/m2eclipse-modello/commit/adfa84b806ac50d6444ced3a8550ecdc0b9b2a1e

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>