iproto: strip prefixes from generated constant strings

There's no need to add prefixes to generated iproto constant strings
(like IPROTO_, IPROTO_FEATURE_, etc) because we strip them anyway when
exporting constants to Lua. Let's drop the prefixes to cleanup the code.
Note that enum constants themselves still have the prefixes to avoid
name clashes.

Follow-up #8443
Follow-up commit b3fb883b ("iproto: export IPROTO constants to Lua
automatically")

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring