It was mistakenly added to that list in D23560 (committed in rL285712). RISCV is an experimental backend and should never have been in that list, I mistakenly interpreted LLVM_ALL_TARGETS as a list of all targets rather than targets to build by default. Unfortunately, because of this the RISCV backend has been building by default when it shouldn't be.
Please see my message to llvm-dev for more info and analysis.