package-cmake: remove now-redundant target ccache support

All the complexity with the different ways that CMAKE_C_COMPILER and
CMAKE_C_COMPILER_ARG1 can be set are no longer needed, it's all handled
by the toolchain wrapper now.

Note that it is still necessary to handle this for the host build.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>