toolchain-external: remove non-existent mips-sf musl toolchains

Since 1.1.6, the mips softfloat toolchains are merged into the mips
toolchain using multilib. Our external toolchain infrastructure copies
the correct version to the target depending on the BR2_SOFT_FLOAT
option.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>