Battery (Linux): remove option --battery-dir

It was added in c5c3cecb93 but there was no comment or issues linked to indicate why it was necessary.

`/sys/class/power_supply` is standardized in <https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-power>. I don't I want to support non-standard platform unless proper reason is given

src/data/help.json

@@ -1026,15 +1026,6 @@
                 "default": "main"
             }
         },
-        {
-            "long": "battery-dir",
-            "desc": "The directory where the battery folders are",
-            "remark": "Linux only",
-            "arg": {
-                "type": "path",
-                "default": "/sys/class/power_supply/"
-            }
-        },
         {
             "long": "battery-use-setup-api",
             "desc": "Set if \"SetupAPI\" should be used on Windows to detect battery info",