#666 remove duplicate param

[valentinsulzer]

Description

Remove duplicate parameter
Fixes #666

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Merging #676 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #676     +/-   ##
=========================================
+ Coverage   98.54%   98.64%   +0.1%     
=========================================
  Files         169      169             
  Lines        8441     9064    +623     
=========================================
+ Hits         8318     8941    +623     
  Misses        123      123

Impacted Files	Coverage Δ
pybamm/expression_tree/vector.py	100% <0%> (ø)	⬆️
pybamm/expression_tree/functions.py	100% <0%> (ø)	⬆️
pybamm/discretisations/discretisation.py	100% <0%> (ø)	⬆️
pybamm/expression_tree/matrix.py	100% <0%> (ø)	⬆️
pybamm/expression_tree/array.py	100% <0%> (ø)	⬆️
pybamm/expression_tree/concatenations.py	98.11% <0%> (+0.06%)	⬆️
pybamm/spatial_methods/spatial_method.py	95.77% <0%> (+0.12%)	⬆️
pybamm/expression_tree/binary_operators.py	98.96% <0%> (+0.52%)	⬆️
pybamm/expression_tree/symbol.py	98.25% <0%> (+0.67%)	⬆️
pybamm/processed_variable.py	97.02% <0%> (+1.5%)	⬆️
... and 1 more

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