Remove the support for BOARD_HAS_EXT4_RESERVED_BLOCKS.

This is only used in file-based OTA, where we may reserve space on
/system partition. With the deprecation of file-based OTA since O, the
code has become obsolete.

Test: `m dist`
Change-Id: I2bd686b292f93b566fe28a2bcd74c564ffc75dac