rpm: Drop override on %_libexecdir

rpm: Drop override on %_libexecdir

We no longer need it, as distributions are converging on /usr/libexec
since FHS 3.0 has specified it as the acceptable path for libexecdir.

Fedora has been using /usr/libexec since the beginning, and
openSUSE has accepted software installed into /usr/libexec since
September 2019 in openSUSE:Factory. openSUSE Leap 15.x still uses
/usr/lib for %_libexecdir, but the rest of the spec should handle the
change gracefully.

Signed-off-by: Neal Gompa <ngompa13@gmail.com>