CLN: remove core/matrix.py, not imported into the pandas namespace
+1
 I don't think this needs to be deprecated in any way.

anyone disagree?

@wesm @jorisvandenbossche 
 I'm +1 on removal. Deprecation warning ought to have been there in 2011 but at this point minimal users will be affected. 
 you pushed the button!! =) 
 oh right
hmm

is there a way to disable it in GitHub?
 No; it's been requested many times by users, though, since in a lot of OSS projects with tighter merge protocol they've had mishaps with merges happening by accident
