[cmake] Delete redundant install command for clang-refactor.

Summary: Install targets for clang tools are controlled by
CLANG_BUILD_TOOLS, and when OFF, cmake issues the following warning:

WARNING: Target "clang-refactor" has EXCLUDE_FROM_ALL set and will not
be built by default but an install rule has been provided for it.
CMake does not define behavior for this case.

Differential Revision: https://reviews.llvm.org/D41807

llvm-svn: 322147