classes/externalsrc: delete symlinks at start of do_buildclean

classes/externalsrc: delete symlinks at start of do_buildclean

To help users find the work and log directories (especially within the
eSDK) we create symlinks to these from the source tree. However during
do_buildclean we call "make clean", and some project Makefiles which
delete something like "*/*/lib" which will match files underneath the
oe-workdir symlink and fail. do_buildclean is called from do_clean which
is in turn called by devtool reset by default, and thus devtool reset is
blocked. An example of a recipe where this is visible is the openssl-qat
recipe in meta-intel.

In order to fix this, delete the symlinks at the start of do_buildclean
since we shouldn't need them at that point anyway, and thus make clean
won't be able to trip over them.

Fixes [YOCTO #11036].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>