doc: remove artifacts of doc build recently added

Document building now copies files from the boards/ and samples/
folders into the doc/ folder, but didn't delete these copies after
the build. Remove the copied folders after the docs are built and
include removing these folders in the make clean actions.

Jira: ZEP-1650

Change-Id: Id5f04a53f0d865bd4211138a35852ac8ab115239
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>