Simplify WSL docs

[traversaro]

In particular:

• Just document WSLg to run graphical applications as not it is available also on Windows 10
• Do not document anymore WSL1 as it is not really widely used, and it is not worth to document it (any user expert enough to run WSL1 will be able to configure its system appropriately)

Fix #1190 .

[traversaro]

For the sake of googlability, I will include here the information on how to use the robotology-superbuild with WSL1, that are removed by this PR.

WSL1

With respect to WSL2, WSL1 uses the same IP address used by the Windows machine, so the amount of configuration and tweaks required are less.

Run graphical applications on WSL1

To run graphical applications on WSL, you need to install a X Server for Windows, that will be able to visualize the windows WSL-based applications, see https://www.howtogeek.com/261575/how-to-run-graphical-linux-desktop-applications-from-windows-10s-bash-shell/ for more info. For information of X Servers that can be installed on Windows, follow the docs in https://github.com/sirredbeard/Awesome-WSL#10-gui-apps .

Sanitize enviroment variables for WSL1

By default, the PATH enviroment variable in WSL will contain the path of the host Windows system, see microsoft/WSL#1640 and microsoft/WSL#1493. This can create problems,
as the CMake in WSL may find (incompatible) Windows CMake packages and try to use them, creating errors due to the compilation.
To avoid that, you can add the following line in the WSL .bashrc that filters all the Windows paths from the WSL's enviromental variables:

for var in $(env | awk {'FS="="} /\/mnt\//{print $1}'); do export ${var}=\"$(echo ${!var} | awk -v RS=: -v ORS=: '/\/mnt\// {next} {print $1}')\" ; done

[traversaro]

Please ignore the CI failure as it is related to #1344 .