of/unittest: Remove obsolete code

Commit 3ce04b4a9, "Removes OF_UNITTEST dependency on OF_DYNAMIC config
symbol" removes a bunch of code, but missed a few minor bits. Clean it
up by removing the node removal cache and flag.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Gaurav Minocha <gaurav.minocha.os@gmail.com>
Cc: Rob Herring <robh@kernel.org>