Remove BTA_SYS_EVT_ENABLED_EVT

It's only initiated from inside bta_sys_main, has a single line
and does not differ in state from BTA_SYS_API_ENABLED_EVT.

Collapse them.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I89fef964cc8a76647d32f278d4389c5c8c066bfc