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
9fa94e1c09 ("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

share/mk/src.opts.mk

@@ -125,7 +125,6 @@ __DEFAULT_YES_OPTIONS = \
     LLVM_ASSERTIONS \
     LLVM_BINUTILS \
     LLVM_COV \
-    LLVM_CXXFILT \
     LOADER_BIOS_TEXTONLY \
     LOADER_GELI \
     LOADER_KBOOT \
@@ -508,12 +507,6 @@ MK_LLVM_COV:= no
 MK_LLVM_BINUTILS:=	yes
 .endif
 
-.if ${MK_LLVM_BINUTILS} == "yes"
-# MK_LLVM_CXXFILT is a subset of MK_LLVM_BINUTILS and should therefore be
-# enabled if MK_LLVM_BINUTILS is set.
-MK_LLVM_CXXFILT:=	yes
-.endif
-
 .if ${MK_LOADER_VERIEXEC} == "no"
 MK_LOADER_VERIEXEC_PASS_MANIFEST := no
 .endif