Revert "userns: Add a Kconfig option to enforce strict kuid and kgid type checks"

This reverts commit ede5ded660b15f3d9e890f720cc3b059fdae2889.
Revert "userns: Add kuid_t and kgid_t and associated infrastructure in uidgid.h"

This reverts commit 5bc8abf36a7f4e755a026919dfc28133d6b00653.