[test] Remove test description in porous medium flow tests

Remove test descriptions in porous medium flow tests to avoid being outdated (fixes #1106 (closed)). The other tests, as well as porousmediumflow/1p/ have already been addressed in !2988 (merged).

Progress through test folders:

• 2p @stefaniekiemle
• 2p1c @stefaniekiemle
• 2p2c @stefaniekiemle
• 2pnc @stefaniekiemle
• 2pncmin @stefaniekiemle
• 3p @IvBu
• 3p3c @IvBu
• 3pwateroil @IvBu
• co2 @IvBu
• mpnc @IvBu
• richards @yue
• richardsnc @yue
• solidenergy @yue
• tracer @yue