linux/common-config.nix: disable LPAE on armv7l-linux

LPAE was enabled to support native armv7l builders running in QEMU on aarch64,
but this option disables support for processors which don't support LPAE, which
are still relatively common. In particular, Beaglebones use the Cortex-A8, which
doesn't support LPAE.

Also, if you attempt to boot an LPAE kernel on a CPU that doesn't support it,
it fails before even earlycon is initialized. This makes the problem difficult
to debug without enabling CONFIG_DEBUG_LL or using a hardware debugger.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -880,8 +880,6 @@ let
       # Keeping it a built-in ensures it will be used if possible.
       FB_SIMPLE = yes;
 
-    } // optionalAttrs (stdenv.hostPlatform.system == "armv7l-linux") {
-      ARM_LPAE = yes;
     };
   };
 in