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
1 job for libexec in 3 minutes and 31 seconds
Status Job ID Name Coverage
  Test
passed #3911303
build_stable

00:03:31