inputflinger: remove "host" code

Confusingly, this is not related to running inputflinger tests etc. on
the host, but a vestige of an architectural plan we had many years ago
but never followed through on. It should be safe to remove now.

Bug: 245989146
Test: Run `m`, check everything builds correctly
Change-Id: I24aa9809ef7d678b0371fb0e868c6298d1d05236