fs_mgr: Remove deprecated mount option

Mount option "nomblk_io_submit" was deleted from
upstream kernel v3.9 in 36ade451a5d736e61ac8302b64aacc5acb5e440f
and has not reappeared since then.
Now it only generates 'Ignoring removed nomblk_io_submit option'
in the logs

Test: manual
Change-Id: Ie609e7d3d38e808eeede1f1c55435ed027726025
diff --git a/fs_mgr/fs_mgr.cpp b/fs_mgr/fs_mgr.cpp
index 5156754..711d7c1 100644
--- a/fs_mgr/fs_mgr.cpp
+++ b/fs_mgr/fs_mgr.cpp
@@ -215,10 +215,6 @@
          */
         if (!(*fs_stat & FS_STAT_FULL_MOUNT_FAILED)) {  // already tried if full mount failed
             errno = 0;
-            if (fs_type == "ext4") {
-                // This option is only valid with ext4
-                tmpmnt_opts += ",nomblk_io_submit";
-            }
             ret = mount(blk_device.c_str(), target.c_str(), fs_type.c_str(), tmpmnt_flags,
                         tmpmnt_opts.c_str());
             PINFO << __FUNCTION__ << "(): mount(" << blk_device << "," << target << "," << fs_type