[cleanup][tests] Remove grid files for 1p2c

*The dgf files are no longer needed, therefore the folder "grids" is removed

test/porousmediumflow/1p2c/implicit/grids/test_1p2c.dgf

-DGF
-Interval
-0 0   % first corner 
-1 1   % second corner
-20 2  % number of cells in x and y direction
-# 
-
-BOUNDARYDOMAIN
-default 1    % all boundaries have id 1
-#BOUNDARYDOMAIN
-# unitcube.dgf 

test/porousmediumflow/1p2c/implicit/grids/test_1p2cniconduction.dgf

-DGF
-Interval
-0 0   % first corner 
-5 1   % second corner
-200 1  % number of cells in x and y direction
-# 
-
-BOUNDARYDOMAIN
-default 1    % all boundaries have id 1
-#BOUNDARYDOMAIN
-# unitcube.dgf 

test/porousmediumflow/1p2c/implicit/grids/test_1p2cniconvection.dgf

-DGF
-Interval
-0 0   % first corner 
-20 1   % second corner
-80 1  % number of cells in x and y direction
-# 
-
-BOUNDARYDOMAIN
-default 1    % all boundaries have id 1
-#BOUNDARYDOMAIN
-# unitcube.dgf