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

shell-completion/bash/meson.build

@@ -11,7 +11,6 @@ if bashcompletiondir == ''
 endif
 
 custom_target(
-        'systemctl',
         input : 'systemctl.in',
         output : 'systemctl',
         command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],