Revert "Always rename an interface to its name specified in config if no NamePolicy= is specified"

This reverts commit 55b6530b.

This commit description says "Always rename an interface to its name specified
in config if no NamePolicy= is specified", but it does much more:
1. It completely changes the meaning of NamePolicy=kernel. Before, it meant that an interface
   with type==NAMEPOLICY_KERNEL would not be renamed. After, the kernel name only works as
   a fallback, if no policy matches.
2. The "if no NamePolicy= is specified" part is not true at all, the interface will be renamed
   according to the specified NamePolicy=.

After 55b6530b, the should_rename() function is named very misleadingly: it is only used
to mean "respect kernel predictable name if no naming policy matches".

Let's revert, and start with a clean slate. This fixes #11436.