osdep: drop atomic fallback

even msvc (which mpv apparently doesn't support) supports C11 atomics
now. no need to carry around fallback with subtle semantic differences.

meson.build

@@ -333,6 +333,9 @@ if features['libdl']
     dependencies += libdl
 endif
 
+# C11 atomics are mandatory but linking to the library is not always required.
+dependencies += cc.find_library('atomic', required: false)
+
 cplugins = get_option('cplugins').require(
     win32 or (features['libdl'] and cc.has_link_argument('-rdynamic')),
     error_message: 'cplugins not supported by the os or compiler!',
@@ -731,13 +734,6 @@ if features['sdl2-gamepad']
     sources += files('input/sdl_gamepad.c')
 endif
 
-stdatomic_dep = cc.find_library('atomic', required: false)
-features += {'stdatomic': cc.has_header_symbol('stdatomic.h', 'atomic_int', dependencies: stdatomic_dep,
-                                               required: get_option('stdatomic'))}
-if features['stdatomic']
-    dependencies += stdatomic_dep
-endif
-
 uchardet_opt = get_option('uchardet').require(
     features['iconv'],
     error_message: 'iconv was not found!',