methodpool: Retire it, remove global method scope

Having a global method scope confuses users and with the introduction
of parallel parsing, its not even possible to correctly detect conflicting
functions. Rather than try and fix that, its simpler to retire the global
method scope and restrict functions to those locations they're defined
within. This is more what users actually expect too.

If we remove the global function scope, the need for methodpool is reduced
to the point we may as well retire it. There is some small loss of caching
of parsed functions but timing measurements so the impact to be neglibile
in the overall parsing time.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Tested-by: Denys Dmytriyenko <denys@ti.com>