ccache.bbclass: don't remove CCACHE_DIR as part of do_clean
Removing the ccache directory as part of do_clean is unnecessarily
conservative and defeats many of the benefits of ccache.
The original justification for this behaviour was to avoid confusion
in the corner case that the ccache directory becomes corrupted.
However the standard approach for dealing with such highly unlikely
corner cases (ie manually removing tmp) would also recover from
corruption of the ccache directories, without the negative impact of
defeating ccache during normal development.
Signed-off-by: Andre McCurdy <armccurdy@gmail.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
