_platform/linux.py: Simplify return statement

The linter dislikes an if/return/else/return where it can be
simplified to a return of the if condition.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

buildstream/_platform/linux.py

@@ -111,8 +111,4 @@ class Linux(Platform):
         except subprocess.CalledProcessError:
             output = ''
 
-        if output == 'root':
-            return True
-
-        else:
-            return False
+        return output == 'root'