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

rules.d/meson.build

@@ -76,7 +76,6 @@ foreach tuple : rules_in
         want = tuple.length() == 1 or tuple[1]
 
         rule = custom_target(
-                tuple[0],
                 input : tuple[0] + '.in',
                 output: tuple[0],
                 command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],