toolchain-external: finish removal of SH2A toolchains

Commit c68c365d ("toolchain-external:
remove CS sh2 toolchains") removed the definitions of the
BR2_TOOLCHAIN_EXTERNAL_CODESOURCERY_SH2A_201103 and
BR2_TOOLCHAIN_EXTERNAL_CODESOURCERY_SH2A_201009, but did not actually
remove the code that was using those options.

So this commit removes the parts of the code that are currently dead
due to this: the definition of the prefix of those toolchains, the
hashes, and the URLs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>