Drop pypy3-dev (#1700)

PyPy has now only one development tip, `default`,
others are version-line-specific maintenance branches.
The py3k branch has been closed in 2016.
As such, a separate pypy3-dev entry no longer makes sense.

plugins/python-build/share/python-build/pypy3-dev

@@ -1,4 +0,0 @@
-#require_gcc
-prefer_openssl11
-install_package "openssl-1.1.0j" "https://www.openssl.org/source/old/1.1.0/openssl-1.1.0j.tar.gz#31bec6c203ce1a8e93d5994f4ed304c63ccf07676118b6634edded12ad1b3246" mac_openssl --if has_broken_mac_openssl
-install_hg "pypy3-dev" "https://bitbucket.org/pypy/pypy" "py3k" "pypy_builder" verify_py32 ensurepip