meson: drop explicit custom_target names

[1] says:
> Since 0.60.0 the name argument is optional and defaults to the basename of
> the first output
We specify >= 0.62 as the supported version, so drop the duplicate name in all cases
where it is the same as outputs[0], i.e. almost all cases.

[1] https://mesonbuild.com/Reference-manual_functions.html#custom_target

sysusers.d/meson.build

@@ -20,7 +20,6 @@ in_files = [['basic.conf',            true],
 foreach tuple : in_files
         file = tuple[0]
         custom_target(
-                file,
                 input : file + '.in',
                 output: file,
                 command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],