scripts/update-symvers: Drop symsets extraction
    
    symsets extraction might be possibly used by SLE11-SP4-LTSS branch.
    The extracted and tracked files are stored in the kernel-source.git and
    they're then used in rpm/kernel-binarin.spec.in as a reference for
    further symsets generation. The generated {main,base,extra}-symsets
    are then used: a) to check against KABI change, b) generation of
    *-symsets structure in resulting RPM with `rpm/symsets.pl --generate-symsets`.
    
    Let's not track -symsets (updates) in kernel-source.git anymore, i.e.
    SLE11-SP4-LTSS RPM will still contain freshly generated artifacts (with
    frozen reference) but we'll never extract them back into
    kernel-source.git with scripts/update-symvers. Assumption is that
    existing symbols don't change and added symbols are added on top of
    reference. Any newer branches are unaffected, they obsoleted -symsets
    long time ago.