Fixes an unused variable warning

26 jobs for mohr/issue142-mc in 195 minutes and 58 seconds (queued for 2 seconds)
Name Stage Failure
failed
coverage Test
	222 - TrilinosVectorTest (Failed)
223 - TrilinosSparseMatrixTest (Failed)
Errors while running CTest
Uploading artifacts for failed job
Uploading artifacts...
WARNING: coverage/coverage.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
msvc-14.2_MpiOnly_Dbg Test
WALBERLA_THREAD_SAFE_LOGGING:BOOL=ON
WALBERLA_VERSION:STRING=5.0
WALBERLA_VS_MULTI_PROCESS_BUILD:BOOL=ON
WARNING_DEPRECATED:BOOL=ON
WARNING_DISABLE:BOOL=OFF
WARNING_ERROR:BOOL=OFF
WARNING_PEDANTIC:BOOL=ON
Cleaning up file based variables
ERROR: Job failed: Process exited with status 1
failed
msvc-14.2_MpiOnly Test
WALBERLA_THREAD_SAFE_LOGGING:BOOL=ON
WALBERLA_VERSION:STRING=5.0
WALBERLA_VS_MULTI_PROCESS_BUILD:BOOL=ON
WARNING_DEPRECATED:BOOL=ON
WARNING_DISABLE:BOOL=OFF
WARNING_ERROR:BOOL=OFF
WARNING_PEDANTIC:BOOL=ON
Cleaning up file based variables
ERROR: Job failed: Process exited with status 1
failed
msvc-14.2_Serial_Dbg Test
WALBERLA_THREAD_SAFE_LOGGING:BOOL=ON
WALBERLA_VERSION:STRING=5.0
WALBERLA_VS_MULTI_PROCESS_BUILD:BOOL=ON
WARNING_DEPRECATED:BOOL=ON
WARNING_DISABLE:BOOL=OFF
WARNING_ERROR:BOOL=OFF
WARNING_PEDANTIC:BOOL=ON
Cleaning up file based variables
ERROR: Job failed: Process exited with status 1
failed
benchmark_clang8 Test
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 404, in _grab_next_args
for seg in self._plot_args(this, kwargs):
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 384, in _plot_args
x, y = self._xy_from_xy(x, y)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 243, in _xy_from_xy
"have shapes {} and {}".format(x.shape, y.shape))
ValueError: x and y must have same first dimension, but have shapes (14,) and (0,)
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
benchmark_gcc9 Test
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 225, in __call__
yield from self._plot_args(this, kwargs)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 391, in _plot_args
x, y = self._xy_from_xy(x, y)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 269, in _xy_from_xy
raise ValueError("x and y must have same first dimension, but "
ValueError: x and y must have same first dimension, but have shapes (14,) and (0,)
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
benchmark_intel19 Test
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 404, in _grab_next_args
for seg in self._plot_args(this, kwargs):
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 384, in _plot_args
x, y = self._xy_from_xy(x, y)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 243, in _xy_from_xy
"have shapes {} and {}".format(x.shape, y.shape))
ValueError: x and y must have same first dimension, but have shapes (14,) and (0,)
Cleaning up file based variables
ERROR: Job failed: exit code 1