Revert "[infra] Make worktrees work (again?)"

This reverts commit a531b78.

Reason for revert: Doesn't work on Windows.

Original change's description:
> [infra] Make worktrees work (again?)
>
> History:
>
> * Back in (pre June) 2018 worktrees in git just worked.
> * https://dart-review.googlesource.com/c/sdk/+/48091 made it not work.
> * #33619 was filed.
> * https://dart-review.googlesource.com/c/sdk/+/127485 was created to
>   work around the problem.
>
> I don't know when it happened, but I can't make the above workaround
> work (at least not anymore) --- adding a `default_git_folder` to my
> `args.gn` has no effect.
>
> This CL builds on it and automatically sets the correct path based on
> a call to `git rev-parse --resolve-git-dir .git` which gives the
> correct dir for both worktrees and non-worktrees.
>
> Change-Id: I4dcf0445a44348621752fb88d4dab4c99c6ddd1d
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/190522
> Reviewed-by: Sigmund Cherem <[email protected]>
> Reviewed-by: William Hesse <[email protected]>
> Commit-Queue: Jens Johansen <[email protected]>

[email protected],[email protected],[email protected],[email protected]

Change-Id: I7681a7b48abdd6e5901f1e445015adc9bf51ef77
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/191361
Reviewed-by: Jens Johansen <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

sdk_args.gni

@@ -18,10 +18,7 @@ declare_args() {
   #
   # to out/ReleaseX64/args.gn. The path above can be extracted from the `.git`
   # file under the git worktree folder.
-  # The script run here should take care of everything automatically though.
-  default_git_folder = exec_script("$_dart_root/tools/get_dot_git_folder.py",
-                                   [ rebase_path("$_dart_root/.git") ],
-                                   "trim string")
+  default_git_folder = "$_dart_root/.git"
 
   # Whether to enable the SDK hash check that will prevent loading a kernel
   # into a VM which was built with a different SDK.