Stop setting SourceCode attribute

[tomhughes]

SCons 4 has dropped dropped support for the SourceCode attribute.

[lightmare]

Not knowing what it was for, I found this note in Scons 3 doc:

scons will, by default, fetch files from SCCS or RCS subdirectories without explicit configuration. This takes some extra processing time to search for the necessary source code management files on disk. You can avoid these extra searches and speed up your build a little by disabling these searches as follows:

env.SourceCode('.', None)

I never knew there was such a default. Is that extra search insignificant, or would it be more appropriate to keep fallback for SCons 3? Something like:

if getattr(env, 'SourceCode', None): env.SourceCode('.', None)