removed physics and added floor collisions

where did you get the <surface>...</surface> tag from?

from the gazebo sdf format:
```
<ode>
Description: ODE friction parameters
Required: 0    Type: n/a
    <mu>
    Description: Coefficient of friction in the range of [0..1].
    Required: 0    Type: double    Default: 1
    <mu2>
    Description: Second coefficient of friction in the range of [0..1]
    Required: 0    Type: double    Default: 1
    <fdir1>
    Description: 3-tuple specifying direction of mu1 in the collision local reference frame.
    Required: 0    Type: vector3    Default: 0 0 0
    <slip1>
    Description: Force dependent slip direction 1 in collision local frame, between the range of [0..1].
    Required: 0    Type: double    Default: 0
    <slip2>
    Description: Force dependent slip direction 2 in collision local frame, between the range of [0..1].
    Required: 0    Type: double    Default: 0
```

thus, a value of 100 (50) does not seem right
what's more, it seems gazebo does not simulate friction...

can you also drive around without the <surface>...</surface> part? if yes, we should remove this for each floor_*