Do not access ext_csd register in debugfs from storaged

Starting from Android R, platform code cannot access files in debugfs
since it does not guarantee ABI stability. This CL removes the code in
storaged that accesses the EMMC's ext_csd register from debugfs as a
backup if it is unable to obtain the information from Health HAL.

Test: build and boot.
Bug: 134669095
Change-Id: I3bfeb3963de5fb5e6bb3eaa9df43baacad164816