Revert "Store private binaries in /usr/lib/polkit-1 instead of /usr/libexec"
This reverts commit 6fbcc6cd.
/usr/libexec is nowadays widely used for internal binaries and documented in the FHS so use that directory.
[1] https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html