Remove cgo_context_data (#4599)

It's tempting to use a single target to centralize cc toolchain lookup,
but ultimately it is incorrect. Toolchain resolution must be performed
by the rules using that toolchain, in order to properly match on the
exec platforms. The other reason for cgo_context_data was that
previously optional toolchains were not possible, but now they are.
Cleanup this incorrect rule to thwart the temptations.

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Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>