depends: Remove _LIBCPP_DEBUG from depends DEBUG mode

It was deprecated in LLVM 15, turned into a compile-time error in LLVM 16:
```bash
In file included from /usr/lib/llvm-16/bin/../include/c++/v1/cassert:19:
/usr/lib/llvm-16/bin/../include/c++/v1/__assert:22:5: error: "Defining _LIBCPP_DEBUG is not supported anymore.
Please use _LIBCPP_ENABLE_DEBUG_MODE instead."
    ^
1 error generated.
```

and has been removed entirely in LLVM 17 (main),
llvm/llvm-project@ff573a4.

Building libc++ in debug mode, will also automatically set
`_LIBCPP_ENABLE_DEBUG_MODE` (the new define), so adding it to depends
doesn't seem useful, and would just result in redefinition errors.

I'm wondering if as a followup, we could enable a DEBUG build of libc++
in our MSAN CI job?

Somewhat related to google/oss-fuzz#9828, where
it looks like we'll have to sort out getting a DEBUG build of LLVM.

depends/hosts/linux.mk

@@ -17,7 +17,7 @@ linux_release_CXXFLAGS=$(linux_release_CFLAGS)
 linux_debug_CFLAGS=-O1
 linux_debug_CXXFLAGS=$(linux_debug_CFLAGS)
 
-linux_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_DEBUG=1
+linux_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC
 
 ifeq (86,$(findstring 86,$(build_arch)))
 i686_linux_CC=gcc -m32