Drop redundant option from %meson

There's no need to pass the --buildtype=plain option to the %meson RPM
macro, because it's one of the default options used by the macro.

There's no need to do a build just for this.

Fallout from ca4846e684

Resolves: #2165742

toolbox.spec

@@ -105,7 +105,6 @@ ln -s src/pkg pkg
 ln -s src/vendor vendor
 
 %meson \
-    --buildtype=plain \
     -Dmigration_path_for_coreos_toolbox=true \
     -Dprofile_dir=%{_sysconfdir}/profile.d \
     -Dtmpfiles_dir=%{_tmpfilesdir}