build: Retire LLVM_CXXFILT option

The LLVM_CXXFILT option was added when we used ELF Tool Chain tools by
default. ELF Tool Chain's c++filt failed to demangle some symbols, so
we added a special case to install LLVM's version and enabled it by
default.

The rest of the LLVM tools are now used by default, as of commit
9fa94e1c099d ("Turn on WITH_LLVM_CXXFILT by default"). Simplify the
build logic by removing a special case and just include llvm-cxxfilt
with the rest of the LLVM utilities.

Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55654