[doc][doxygen][img] cleanup tpfa and mpfa figures
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What this MR does / why does DuMux need it:
As stated in the dumux-course issue #77, there were two points that need to be fixed for the tpfa and the mpfa figure:
Notes for the reviewer
TODO: insert text here
Before you request a review from someone, make sure to revise the following points:
CHANGELOG.md
.\n
character. Make sure there is no \ No newline at end of file
comment in "Changes" of this MR.