Remove system call to "headless" unbundled chooser

The behavior of proxying through a headless implementation of
the unbundled component was left over from an earlier stage of
the migration ("phase 2" of go/sharesheet-unbundling-phases).

This headless use is no longer supported in the unbundled
component, and it seems that some users are triggering this
code path even when unbundling is disabled -- resulting in
displaying both the "legacy" (system-side) UI and then, when
a share target is selected, the new unbundled UI (which is
no longer "headless").

Bug: 213348070
Change-Id: I146b42e9624d795cfa6689b6a417778ae912d8b5
Test: manual verification of crash fix, atest ChooserActivityTest