[Driver] Remove unlikely-working Minix.cpp and Contiki.cpp

Minix is a ToolChain that was added back in 2010 but has been
unmaintained with no test. The constructed command line contains
/usr/gnu/include/c++/4.4.3 and CompilerRT-Generic which are unlikely
working for a long time.

Contiki is a barebone ToolChain that just enables safestack.
This doesn't justify a new ToolChain.

Remove these ToolChains so that their target triples will use Generic_ELF instead.

If these developers feel like having an updated llvm-project is useful,
fixing other build issues and adding a new ToolChain is much better than
having the unmaintained ToolChains.

Reviewed By: brad

Differential Revision: https://reviews.llvm.org/D158461