lib: remove pure attribute from functions that modify memory

lib: remove pure attribute from functions that modify memory

Almost all functions currently marked with pure attribute acquire a
route_node lock. By marking them pure we allow compiler to optimize the
code and not call them when it already knows the return value. This is
completely incorrect.

Only two of eleven functions can be marked as pure. And they still won't
be optimized because they are never called from the same function twice.
Let's remove the ext_pure macro completely to reduce the chance of
repeating this mistake in the future.

Fixes #8866, #8809, #8595, #6992.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>