meson: drop libdl, threads, and librt dependencies

Our baseline glibc is 2.34, which merged libdl, libpthread (the
dependency('threads') target), and librt into libc. Empty .so/.a stubs
remain for backward compatibility with old binaries, but new builds
resolve dl_*, pthread_*, mq_*, timer_*, etc. directly from libc.
On musl the same libraries are likewise empty stubs.

Drop the libdl, threads, and librt entries from every meson.build, and
remove the now-stale 'Libs.private: -lrt -pthread' from libudev.pc.in
since both flags resolve to empty link-time stubs on glibc 2.34+ and
musl.

Verified with readelf -d that libsystemd.so, libudev.so, and systemd no
longer carry DT_NEEDED entries for libdl/libpthread/librt.

src/userdb/meson.build

@@ -5,7 +5,6 @@ executables += [
                 'name' : 'systemd-userwork',
                 'conditions' : ['ENABLE_USERDB'],
                 'sources' : files('userwork.c'),
-                'dependencies' : threads,
         },
         libexec_template + {
                 'name' : 'systemd-userdbd',
@@ -14,13 +13,11 @@ executables += [
                         'userdbd-manager.c',
                         'userdbd.c',
                 ),
-                'dependencies' : threads,
         },
         executable_template + {
                 'name' : 'userdbctl',
                 'conditions' : ['ENABLE_USERDB'],
                 'sources' : files('userdbctl.c'),
-                'dependencies' : threads,
         },
 ]