backends: Remove @PRIVATE_OUTDIR_(ABS)@ substitution
backends: Remove @PRIVATE_OUTDIR_(ABS)@ substitution

It is not documented and does not seems to be used anywhere.

pexip/meson,QuLogic/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,pexip/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson