Move PLYMOUTH_SOCKET define to def.h and nuke plymouth-util.h

Let's not have a file with a single define.

src/basic/def.h

@@ -58,3 +58,8 @@
         STRV_MAKE(CONF_PATHS(n))
 
 #define HIGH_RLIMIT_MEMLOCK (1024ULL*1024ULL*64ULL)
+
+#define PLYMOUTH_SOCKET {                                       \
+                .un.sun_family = AF_UNIX,                       \
+                .un.sun_path = "\0/org/freedesktop/plymouthd",  \
+        }

src/basic/meson.build

@@ -162,7 +162,6 @@ basic_sources = files('''
         parse-util.h
         path-util.c
         path-util.h
-        plymouth-util.h
         prioq.c
         prioq.h
         proc-cmdline.c

src/basic/plymouth-util.h

@@ -1,7 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1+ */
-#pragma once
-
-#define PLYMOUTH_SOCKET {                                       \
-                .un.sun_family = AF_UNIX,                       \
-                .un.sun_path = "\0/org/freedesktop/plymouthd",  \
-        }