[Room Avatar] Remove the hexagonal shape on the room avatar

[giomfo]

Before going further on the room avatar shape in Tchap: https://github.com/tchapgouv/tchap-design/issues/61

We will stop using the hexagonal profile on room avatar in the rooms list. All the avatar will be rounded (like the DM currently)

I would like to remove this hexagonal shape from the room creation screen too

I let you decide to remove totally the code related to the hexagonal profile, or to keep it in the project. I would vote to remove it, we may get it back from your PR

[giomfo]

we voted to remove the related the hexagonal profile for the code

[giomfo]

tested