adbd: Remove explicit capability bounding set code.

Minijail supports dropping capabilities from the bounding set.
Use that code instead.

This cleans up one small loose end in the work started with
https://android-review.googlesource.com/#/c/187775/

Final diff: fewer lines, more unit-tested code.
"""
$ git diff --stat 52cf4fa6aa3e8d13f61b1 adb/daemon/main.cpp
 adb/daemon/main.cpp | 56
 1 file changed, 27 insertions(+), 29 deletions(-)
"""

Bug: 27314096
Change-Id: I238975920b85fae376522eb26f5ad6d9fee0762a