Revert "Don't mount /nix/store ro if it's a mountpoint"

Now that nixUnstable supports remounting in the "/nix/store is a
mountpoint" case, this is no longer necessary.

This reverts commit f1d48aec43.

Signed-off-by: Shea Levy <shea@shealevy.com>

modules/system/boot/readonly-mountpoint.c

@@ -8,7 +8,7 @@ int main(int argc, char ** argv) {
 		fprintf(stderr, "Usage: %s PATH", argv[0]);
 		exit(2);
 	}
-	if (statvfs(argv[1], &stat) != 0) {
+	if(statvfs(argv[1], &stat) != 0) {
 		perror("statvfs");
 		exit(3);
 	}

modules/system/boot/stage-2-init.sh

@@ -51,7 +51,7 @@ echo "booting system configuration $systemConfig" > /dev/kmsg
 chown 0:30000 /nix/store
 chmod 1775 /nix/store
 if [ -n "@readOnlyStore@" ]; then
-    if ! mountpoint /nix/store; then
+    if ! readonly-mountpoint /nix/store; then
         mount --bind /nix/store /nix/store
         mount -o remount,ro,bind /nix/store
     fi