buffer: remove Buffer#toArrayBuffer()

A recent change to v8's API now makes it impossible to memcpy to a
v8::ArrayBuffer without causing it to be externalized. This means that
the garbage collector will not automatically free the memory when the
object is collected.

When/If the necessary API is included to allow the above
Buffer#toArrayBuffer() will be reintroduced.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>