Simplify gpg verification of Source0
    
    The verification was simplified slightly in 903d8f3 ("Remove EL-5 and
    old Fedora conditionals", 2017-07-22).
    
    Further simplifications:
    
        - do away with unneeded variables
        - drop '--batch' and '>/dev/null' from gpg2 --dearmor
        - check tarball signature via stdin
    
    The "noisy output from GnuPG 2.0" alluded to on EL <= 7 is no longer
    present.  This has been tested in mock for el6, el7, and fedora
    releases.