lang/pypy: simplify port

• Use upstreams directory layout instead of FreeBSD's preferred directory layout [1]
• Move the pypy installation into $PREFIX/pypy-x.y
• Remove the ability to build multiple instances (i.e. default to just the JIT instance)
• Install the binary as pypy (instead of pypy-2.6 with a symlink to pypy)
• Remove the creation of cffi modules (will be outsourced to other ports, a la cpython)
• Remove sqlite3 and gdbm dependencies (i.e. cffi modules)

PR: 183795
Differential Revision: https://reviews.freebsd.org/D3209