net: ipa: get rid of empty IPA functions

There are place holder functions in the IPA code that do nothing.
For the most part these are inverse functions, for example, once the
routing or filter tables are set up there is no need to perform any
matching teardown activity at shutdown, or in the case of an error.

These can be safely removed, resulting in some code simplification.
Add comments in these spots making it explicit that there is no
inverse.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>