Remove opencv package from requirements.txt

Opencv packaging is at fault: opencv package installs without
the metadata (egg). Therefore pip installs can't find it.
Still works with conda-based installs.

requirements.txt

 setuptools
 bob.extension
 numpy
-opencv
+# opencv  We need opencv, but the opencv package is broken and can't be installed with pip. Install it with conda.
 bob.io.image
-bob.ip.color
\ No newline at end of file
+bob.ip.color