Remove unnecessary conditional imports

In this pull requests some conditional imports are removed. They are no longer necessary (as numpy imports are protected by try/except since https://github.com/nltk/nltk/commit/73906dcce7e1552de277dcfdcca30e358f433319). 

This should make exception messages more informative and prevent issues like #368 (or http://stackoverflow.com/questions/15320894/python-importerror-maxentclassifier/15321784#15321784 ).

This change doesn't break any tests for me (including tests in environments without numpy & friends).