kconfig: remove unneeded directory generation from local*config

Commit 17263baf ("kconfig: Create include/generated for
localmodconfig") added the 'mkdir' line because local{yes,mod}config
ran streamline_config.pl followed by silentoldconfig at that time.

Since commit 81d2bc22 ("kconfig: invoke oldconfig instead of
silentoldconfig from local*config"), no sub-directory is required.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>