remove new method in UimaContext interface giving unneeded access to external override parameters

The user-facing API (for annotation and UIMA Application programmers) has no need to change for the new External Parameter Override feature because the effect of this should be transparent to these users.

Interfaces in UIMA should remain stable where possible, from release to release, to reduce adoption effort for new releases.

Therefore, remove the new method added to the UimaContext interface to access an override parameter by its override name. After this removal, the UimaContext will have no changes in this release (a good thing).