Revert "meson: Handle the b_lto option as a string option for newer meson versions"
Revert "meson: Handle the b_lto option as a string option for newer meson versions"

This reverts commit 920079edb1ead2f2cbeaf46fd6643ebb344b8b6b.

Upstream meson reverted the breaking change, see
https://github.com/mesonbuild/meson/issues/7493#issuecomment-729020325,
so currently the forward-compatibility code was producing warnings
in builds with newer meson instead.

videolan/dav1d