Cleanup NOTIFICATION_MEMORY_LOGGING_ENABLED

This flag has been enabled since T and the code underneath it is stable.
The actual logging is controlled via StatD (and thus we actually had two
flags for this one feature).

Bug: 269599781
Test: n/a
Flag: EXEMPT flag cleanup
Change-Id: Iec230eefd02cf44c1ba1e27ad0e4b961be009bfd