mk: remove -c and -x commands (clean/xgcc-clean)

the user never runs these. it's not needed. the design
of lbmk already cleans everything, or caches everything,
between builds. everything cached out of tree.

it's not expected to ever need to clean manually, so just
remove the option. knobs are for nobs.

releases don't need it either, because we tar everything
before building, thus eliminating all build artifacts.
we do this on a fresh copy of lbmk on ./mk release

Signed-off-by: Leah Rowe <leah@libreboot.org>

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