Remove legacy bubble bar flag

Sometimes people are flipping this instead of the aconfig flag
which isn't ideal & it seems like there's a crash from
FeatureFlagsClassicDebug when this is toggled.

Flag: EXEMPT removing obsolete flag
Test: make TheFlippinApp; install and check that "bubble bar"
      isn't visible in classic flags (with other CL)
Bug: 374692464
Bug: 256873975
Change-Id: I49b7dd88567d2e9defee08469909ae4f814c9fb3