Remove LLVM_INSTALL_CCTOOLS_SYMLINKS

It used to symlink dsymutil to llvm-dsymutil, but after r327790 llvm's dsymutil
binary is now called dsymutil without prefix.

r327792 then reversed the direction of the symlink if
LLVM_INSTALL_CCTOOLS_SYMLINKS was set, but that looks like a buildfix and not
like something anyone should need.

https://reviews.llvm.org/D45966

llvm-svn: 330727