Revert of Default-enable external startup data for Linux for stand-alone...

Revert of Default-enable external startup data for Linux for stand-alone builds. (patchset #1 id:1 of https://codereview.chromium.org/956373002/)

Reason for revert:
The CL caused two issues:
- a weird build issue on V8 mips builder
- d8 cannot be invoked via PATH, since then it doesn't find its external snapshot.

The 2nd issue might even be WAI, but this needs more consideration.

Original issue's description:
> Default-enable external startup data for Linux for stand-alone builds.
>
> Notes:
> - Other platforms to follow later.
> - This follows Chromium practice, that mostly uses this feature these days.
> - The statically linked-in startup data will stay. So whoever prefers
>   the old way just needs to set the flag differently.
>
> Reland crrev.com/959693002, once crrev.com/960883003 is in.
>
> R=machenbach@chromium.org
> BUG=
>
> Committed: https://crrev.com/a0bdb103b676b4c7fa6b9f2e7149e716549c05d1
> Cr-Commit-Position: refs/heads/master@{#26980}

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

Review URL: https://codereview.chromium.org/981463005

Cr-Commit-Position: refs/heads/master@{#26986}

build/standalone.gypi

@@ -152,11 +152,6 @@
       }, {
         'clang%': 0,
       }],
-      ['(OS=="linux")', {
-        # Gradually roll out v8_use_external_startup_data.
-        # Should eventually be default enabled on all platforms.
-        'v8_use_external_startup_data%': 1,
-      }],
     ],
     # Default ARM variable settings.
     'arm_version%': 'default',