oeqa/selftest: improve config writing and cleanup

oeqa/selftest: improve config writing and cleanup

The selftest.inc configuration file is deleted in both tearDown() and setUp() so
there's no need to use addCleanup() to remove statements from it.

Use write_config instead of append_config if the intention is to start from an
empty config file, for clarity.

Finally remove some misleading comments that claim that append_config() writes
to local.conf when it doesn't.

Signed-off-by: Ross Burton <ross.burton@intel.com>