Use the default `buildtype` in Meson config

The Meson build system sets a default_argument of buildtype=release. This was done with the idea that it would make the default build type work the same as with CMake.

However, after some recent discussions with the Meson team, and with recent changes starting in Meson 1.8 to how the buildtype default_argument is processed, I think it is a bad idea for us to set this. See also the conversation in https://github.com/mesonbuild/wrapdb/issues/2208