remove link to pdf doc on documentation (#415)

Co-authored-by: Jaquier Aurélien Tristan <[email protected]>

docs/source/eFeatures.rst

@@ -4,9 +4,6 @@
 eFeature descriptions
 =====================
 
-A pdf document describing the eFeatures is available
-`here <http://bluebrain.github.io/eFEL/efeature-documentation.pdf>`_.
-
 Time, voltage and current (if given) are interpolated using `interp_step` setting (default `interp_step = 0.1` ms) before efeatures are extracted from them.
 Since, they are technically features in eFEL, you can retrieve the interpolated time, voltage and current (if given), like any other feature.