meson: remove strange dep that causes meson to enter infinite loop

The value is obviously bogus, but didn't seem to cause problems so far.
With meson-0.52.0, it causes a hang. The number of aliases is always rather
small (usually just one or two, possibly up to a dozen in a few cases), so
even if this causes some looping, it is strange that it has such a huge impact.
But let's just remove it.

Fixes #13742.

Tested with meson-0.52.0-1.module_f31+6771+f5d842eb.noarch,
meson-0.51.1-1.fc29.noarch.

man/meson.build

@@ -68,7 +68,6 @@ foreach tuple : xsltproc.found() ? manpages : []
                 foreach htmlalias : htmlaliases
                         link = custom_target(
                                 htmlalias,
-                                input : p2,
                                 output : htmlalias,
                                 command : ['ln', '-fs', html, '@OUTPUT@'])
                         if want_html