Remove senaite.api import

[juangallostra]

Description of the issue/feature this PR addresses

Linked issue: #810

Current behavior before PR

Travis fails when trying to import senaite api with the following traceback:

File "/home/travis/build/senaite/senaite.core/bika/lims/browser/sample/__init__.py", line 13, in <module>
   from .printform import SamplesPrint
 File "/home/travis/build/senaite/senaite.core/bika/lims/browser/sample/printform.py", line 15, in <module>
   from senaite import api
ZopeXMLConfigurationError: File "/home/travis/build/senaite/senaite.core/bika/lims/configure.zcml", line 29.2-29.32
   ZopeXMLConfigurationError: File "/home/travis/build/senaite/senaite.core/bika/lims/browser/configure.zcml", line 41.2-41.42
   ImportError: No module named senaite

Desired behavior after PR is merged

Travis is happy again (Travis is hard to please)

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.