net: ipa: get rid of empty GSI functions

There are place holder functions in the GSI code that do nothing.
Remove these, knowing we can add something back in their place if
they're really needed someday.

Some of these are inverse functions (such as teardown to match setup).
Explicitly comment that there is no inverse in these cases.

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