dev-python/msgpack: remove requirements.txt sed

This release does not contain this file, and will fail to build because
of it. Removing the sed allows a proper build on my machine.

Closes: https://bugs.gentoo.org/650064
Closes: https://github.com/gentoo/gentoo/pull/7411
Package-Manager: Portage-2.3.24, Repoman-2.3.6

dev-python/msgpack/msgpack-0.5.6.ebuild

@@ -33,8 +33,6 @@ python_prepare_all() {
 	if ! use native-extensions ; then
 		sed -i -e "/have_cython/s:True:False:" ./setup.py || die
 	fi
-	# make sure cython is optional
-	sed -i '/^Cython/d' requirements.txt || die
 	distutils-r1_python_prepare_all
 }