bootstat: Remove the CAP_SYSLOG capability

This change removes the CAP_SYSLOG file based capability from bootstat,
since the intention is that it should not be accessing the logs in the
long term. In order to avoid bitrot, the fallback code that depends on
CAP_SYSLOG has also been removed.

Bug: 62845925
Test: system/core/bootstat/boot_reason_test.sh
Change-Id: I899be44ef3ac1c4d81072f801d55c928ae09bb15