system/core: replace EVENT_TAG_MAP_FILE with NULL

NULL represents system default.  In the future, NULL could represent
static and dynamic tags, which can come from multiple files based on
implementation details in the liblog library.

Test: gTest logd-unit-tests & liblog-unit-tests
Bug: 31456426
Change-Id: I0e3d296de81ca299ae63d7b83781639ee67ec298