Remove unnecessary branch for BOARD_HAS_EXT4_RESERVED_BLOCKS.

The branch exist for file based OTA, but it is deprecated in O.
Test: m dist

Change-Id: I24275b550f89c2541ef8893fdeffc06f5da01f6e
Merged-In: I24275b550f89c2541ef8893fdeffc06f5da01f6e
(cherry picked from commit 5cf81a3c)