Remove the extern declaration of sehandle from roots.cpp.

It has become obsolete since we replaced the call to make_ext4 with
e2fsprogs (commit ded2dac082, which landed
into P).

Test: mmma -j bootable/recovery
Change-Id: I09141322874213dcb0f1280bba239376e71a4d17

roots.cpp

@@ -51,8 +51,6 @@ using android::fs_mgr::ReadDefaultFstab;
 
 static Fstab fstab;
 
-extern struct selabel_handle* sehandle;
-
 void load_volume_table() {
   if (!ReadDefaultFstab(&fstab)) {
     LOG(ERROR) << "Failed to read default fstab";