* macrotab.h (new_macro_table): Document that removing information

* macrotab.h (new_macro_table): Document that removing information
from an obstack/bcache-managed macro table leaks memory.
* macrotab.c (macro_free, macro_bcache_free): Instead of asserting
that data is never freed in obstack/bcache-managed macro tables,
just leak the storage.
(macro_undef): If we're undefining a macro at exactly the same
source location that we defined it, simply remove the definition
altogether.