meson: simplify video-codecs option parsing

The extra `all_free` that now remains in the list will be ignored in the
loop below anyway so there is no need to have complex code to try to
remove it.

This also means it becomes possible to set things like
`-D video-codecs=all_free,vc1dec`.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33949>

meson.build

@@ -386,13 +386,7 @@ all_codecs = patent_codecs + free_codecs
 if _codecs.contains('all')
   _codecs = all_codecs
 elif _codecs.contains('all_free')
-  selected_codecs = _codecs
-  _codecs = free_codecs
-  foreach c : patent_codecs
-    if selected_codecs.contains(c)
-      _codecs += c
-    endif
-  endforeach
+  _codecs += free_codecs
 endif
 foreach c : all_codecs
    pre_args += '-DVIDEO_CODEC_@0@=@1@'.format(c.to_upper(), _codecs.contains(c).to_int())