Remove get_self_dir()

It didn't serve much value, and was only used twice.
Path(__file__).parent is a pretty easy invocation to use.

Signed-off-by: Tristan Partin <tristan@neon.tech>

test_runner/fixtures/utils.py

@@ -42,11 +42,6 @@ from fixtures.common_types import TimelineId
 Fn = TypeVar("Fn", bound=Callable[..., Any])
 
 
-def get_self_dir() -> Path:
-    """Get the path to the directory where this script lives."""
-    return Path(__file__).resolve().parent
-
-
 def subprocess_capture(
     capture_dir: Path,
     cmd: List[str],