src: remove Buffer::Data(Persistent<T>&)

It hits a compiler bug in gcc <= 4.4 similar to the issue that was
recently addressed in commit 157d2bc:

    ../deps/v8/include/v8.h: In function ‘char*
    node::Buffer::Data(v8::Persistent&) [with TypeName = v8::Object]’:
    ../src/node_crypto.cc:1123: instantiated from here
    ../deps/v8/include/v8.h:876: error: ‘class v8::Data’ is not a
    function,
    ../src/node_internals.h:356: error: conflict with ‘template char*
    node::Buffer::Data(v8::Persistent&)’
    ../src/node_internals.h:357: error: in call to ‘Data’

Remove the helper function, it was only used in a couple of places.
Should fix the build on Ubuntu 10.04.

Fixes #5844.