fs_mgr: overlayfs earlier kernels do not need override_creds=off

Kernels at or older than 4.6, the overlayfs drivers do not need,
or support, override_creds=off.  Drop using the mount option.

Commit 3fe6e52f062643676eb4518d68cee3bc1272091b introduced the
need for override_creds=off, and none of the kernels that contain
this patch without the patch that introduces override_creds can
be used with Android.

Test: compile
Bug: 118882257
Bug: 109821005
Change-Id: I832c8ca3fce0269bdef4ce988541adb7ba9662ed