Revert of Re-land: Enable external startup by default on Linux. (patchset #3 id:40001 of https://codereview.chromium.org/1041683002/)

Reason for revert:
Revert, due to problems with stale build artefacts uncovered by this.

Details: https://code.google.com/p/v8/issues/detail?id=4171

Original issue's description:
> Re-land: Enable external startup by default on Linux.
>
> crrev.com/1016603004 should have fixed the issues that broke this last time.
>
> Notes:
> - The current functionality is meant to stay. If you prefer the old way,
>   just set v8_use_external_startup_data=0.
> - You might notice this if you call d8 via PATH. Either explicitly tell d8
>   where your external snapshot/built-ins are, or change the build options
>   as above.
> - This follows Chromium practice, that mostly uses this feature these days.
> - Other platforms to follow later.
>
> BUG=
>
> Committed: https://crrev.com/bb9c774c164c3552b67b4f1b60809852a44f2ecd
> Cr-Commit-Position: refs/heads/master@{#28858}

TBR=jochen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1177883003

Cr-Commit-Position: refs/heads/master@{#28929}