[CMake] Remove target to build native tablegen

This was once needed so that multiple tablegen binaries don't compile
the library concurrently. However, this isn't needed anymore since
adding USES_TERMINAL to the custom_command.
This is supported by the fact that the target was only building
LLVMSupport since some cleanups a year ago. If this dependency had
really been needed, we would have seen complaints.

Differential Revision: https://reviews.llvm.org/D39299

llvm-svn: 317695