tools: delete v8_external_snapshot.gypi

Delete the v8_external_snapshot target from gyp and disable the
v8_use_external_startup_data option since it is never used anyway.

Refs: https://github.com/nodejs/node/pull/29363#issuecomment-526103293
Fixes: https://github.com/nodejs/node/issues/28964

PR-URL: https://github.com/nodejs/node/pull/29369


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>