Remove unsued 'gtest-sources.cc'

Change-Id: Ib8362928456789a6d57c8618c6466e516ecffc56
Reviewed-on: http://review.couchbase.org/44205
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>