remove Haiku support

Haiku support has been unmaintained for many years, and this issue has
been open for more than 5 years, but apparently the Haiku people have
lost interest:

 https://github.com/MusicPlayerDaemon/MPD/pull/183

Haiku support was therefore deprecated by this commit 4 years ago:
7de8fd04a4 - but in those 4 years, nobody stepped up to adopt
maintainership.

I don't have any computer (or VM) with Haiku and there is no CI with
Haiku support, so I'm unable to adapt the Haiku specific code to API
changes.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/182
Closes https://github.com/MusicPlayerDaemon/MPD/issues/185

meson.build

@@ -158,7 +158,6 @@ is_linux = host_machine.system() == 'linux'
 is_android = get_option('android_ndk') != ''
 is_darwin = host_machine.system() == 'darwin'
 is_windows = host_machine.system() == 'windows'
-is_haiku = host_machine.system() == 'haiku'
 
 if is_android
   common_cppflags += '-DANDROID'
@@ -516,13 +515,6 @@ if is_android
     declare_dependency(sources: [classes_jar]),
     java_dep,
   ]
-elif is_haiku
-  target_type = 'executable'
-  target_name = 'mpd.nores'
-  link_args += [
-    '-lnetwork',
-    '-lbe',
-  ]
 else
   target_type = 'executable'
   target_name = 'mpd'
@@ -562,25 +554,13 @@ mpd = build_target(
   ],
   link_args: link_args,
   build_by_default: not get_option('fuzzer'),
-  install: not is_android and not is_haiku,
+  install: not is_android
 )
 
 if is_android
   subdir('android/apk')
 endif
 
-if is_haiku
-  subdir('src/haiku')
-  custom_target(
-    'mpd',
-    output: 'mpd',
-    input: [mpd, rsrc],
-    command: [addres, '@OUTPUT@', '@INPUT0@', '@INPUT1@'],
-    install: true,
-    install_dir: get_option('bindir'),
-  )
-endif
-
 configure_file(output: 'config.h', configuration: conf)
 
 if systemd_dep.found()