Don't send executionContexts for Clear-Site-Data

Don't send executionContexts for Clear-Site-Data

There are plans to remove executionContexts from the spec: https://github.com/w3c/webappsec-clear-site-data/issues/59

Firefox already removed it https://bugzilla.mozilla.org/show_bug.cgi?id=1548034

Chromium implementation is not finish: https://bugs.chromium.org/p/chromium/issues/detail?id=898503&q=clear-site-data&sort=-modified&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>