Remove 'pip install -e .' command (not necessary with skipsdist=False)

Author: dnerini

tox.ini

@@ -46,7 +46,6 @@ setenv =
     PROJ_LIB={envdir}/share/proj
 commands =
     python {toxinidir}/scripts/create_pystepsrc_tox.py
-    pip install -e .
     pytest --pyargs pysteps --cov=pysteps -ra --disable-warnings