remove obsolete Boost dependency

Boost makes building a piece of software much more difficult than
necessary.  It's a huge library, and just uncompressing it takes a
considerable amount of time. MPD only used a tiny fraction of it, yet
its header bloat made the MPD build very slow.  Locating Boost was
difficult due to its arcane build system and its resistance to use
pkg-config; it's always a special case.  MPD could never use features
of newer Boost versions because Linux distributions always shipped old
Boost versions.  Boost made everything complicated and slow.

So, finally, after getting rid of GLib (commit ccdb94b06c), switching
to C++ and using Boost (commit 0801b3f495), we've finally get rid of
it 8 years later.

Unfortunately, I had to reimplement parts of it along the way
(e.g. IntrusiveList).  Kind of NIH, but on the other hand, compiling
MPD has become much easier for users.

doc/user.rst

@@ -60,7 +60,6 @@ In any case, you need:
 * a C++20 compiler (e.g. GCC 10 or clang 11)
 * `Meson 0.56.0 <http://mesonbuild.com/>`__ and `Ninja
   <https://ninja-build.org/>`__
-* Boost 1.58
 * pkg-config 
 
 Each plugin usually needs a codec library, which you also need to
@@ -95,7 +94,6 @@ For example, the following installs a fairly complete list of build dependencies
       libsqlite3-dev \
       libsystemd-dev \
       libgtest-dev \
-      libboost-dev \
       libicu-dev \
       libchromaprint-dev \
       libgcrypt20-dev