Revert "input: allow input.conf bindings to be declared as builtin"

[guidocella]

This reverts commit 70ff543.

This never did anything because mp_input_load_config() always already called parse_config() with builtin = true for builtin_input_conf. And we already have compatibility input.conf files without depending on this line. So unless someone can decipher what wm4 meant, it can be removed.

[na-na-hi]

This never did anything because mp_input_load_config() always already called parse_config() with builtin = true for builtin_input_conf.

This is incorrect. For key bindings loaded from config files rather than builtin_input_conf, this makes the bindings treated as builtin so they have lower priority compared to non-builtin bindings.

[guidocella]

Ok but we don't have any such config file with that line.

[na-na-hi]

It can be defined in user's config files.

[guidocella]

But why would someone do that? It's not even documented.

[na-na-hi]

But why would someone do that?

It has valid uses. For example, a mpv plugin or script can redefine builtin key bindings, without needing to recompile mpv, while still allowing user bindings in input.conf take preference. It allows changing player key bindings in a less agressive way.