cmGlobalNinjaGenerator: Remove unused member

The `DisableCleandead` member was added by commit c5011399c5 (Ninja:
Avoid cleandead with dyndep bindings for Fortran module dependencies,
2020-11-10, v3.19.0~15^2) but has not been used since commit 1144d25094
(Merge branch 'backport-ninja-no-cleandead' into ninja-no-cleandead,
2020-12-14, v3.19.2~4^2).  Remove it.