Subplugin register/unregister refactored.

Subplugin register/unregister refactored.

A plugin, "filter" and "decoder" no more have their own
internal data structure to store subplugins.

Both filter and decoder subplugins now use APIs of
nnstreamer_subplugin via
nnstreamer_plugin_api.h (filters)
tensordec.h (decoder)

Decoders are to be refactored further to use
nnstreamer_plugin_api.h after moving decoder subplugins
to ext/

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>