Remove system-asio build option

Asio has been shipping with `pkg-config` support since version 1.23, so
this is no longer needed.

meson.build

@@ -22,7 +22,6 @@ project(
 is_64bit_system = build_machine.cpu_family() not in ['x86', 'arm']
 with_bitbridge = get_option('bitbridge')
 with_clap = get_option('clap')
-with_system_asio = get_option('system-asio')
 with_winedbg = get_option('winedbg')
 with_vst3 = get_option('vst3')
 
@@ -225,26 +224,7 @@ endif
 # These are all headers-only libraries, and thus won't require separate 32-bit
 # and 64-bit versions
 
-# NOTE: The standalone asio library does not come with a pkgconfig or CMake
-#       build definition, and Meson thus also won't be able to detect it this
-#       way. As a workaround for distro packaging, configuring the project with
-#       `-Dsystem-asio=true` will use `<asio.h>` from the standard include
-#       directories instead.
-if with_system_asio
-  if not meson.get_compiler('cpp', native : true).check_header('asio.hpp')
-    error('The \'system-asio\' build option was set, but <asio.hpp> was not found')
-  endif
-
-  asio_version = meson.get_compiler('cpp', native : true).get_define('ASIO_VERSION', prefix : '#include <asio.hpp>')
-  if asio_version.to_int() < 102800
-    error('Expected version 1.28.0 of the asio library or higher, found @0@ (MMmmrr)'.format(asio_version))
-  endif
-
-  # This is a dummy dependency, since the library is only accessible implicitly through the system include path
-  asio_dep = declare_dependency()
-else
-  asio_dep = dependency('asio', version : '>=1.28.0')
-endif
+asio_dep = dependency('asio', version : '>=1.28.0')
 
 if meson.version().version_compare('>=0.60')
   # Bitsery's CMake build definition is capitalized for some reason