Avoid adding the STEAMWORKS_NET define symbol to non-standalone platorms

[sandolkakos]

Hi, I've added some logic to avoid the STEAMWORKS_NET scripting define symbol being automatically added to unsupported platforms like Android, iOS, etc.

In these platforms, if STEAMWORKS_NET is found in the defines, it will be removed since it will not work anyway.

In that case, because the console will already display a compiling error, you have to restart Unity in Non-SafeMode to the STEAMWORKS_NET to get automatically removed.

Explanation:

In my case, I do not like spreading #if STEAMWORKS_NET over my code base. I do it only in some special cases.

Scripts with a lot of Steam SDK library usage are part of an AssemblyDefinition like the one below:

[sandolkakos]

This was fixed by #671
And also I've created this other PR with a setting to avoid it from adding the defines at all: #712