ccache.conf: Remove obsolete configuration option

ccache.conf: Remove obsolete configuration option

Since ccache version 4.0, according to
https://github.com/ccache/ccache/blob/master/doc/NEWS.adoc#ccache-40

 * An appropriate cache directory level structure is now chosen automatically. The cache_dir_levels (CCACHE_NLEVELS) configuration option has therefore been removed.

Therefore remove the option which has not been supported by ccache
recipe version since Yocto Hardknott.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>