vala: remove unconditional work around for clang 16 function pointer errors

Hook accumulates the flag over sereval executions as observed in:

- https://github.com/NixOS/nixpkgs/issues/301592
- https://github.com/NixOS/nixpkgs/pull/252484#issuecomment-2089095268

Since this has been committed, gcc started emitting a warning on
`-Wno-incompatible-function-poniter-types` being an unrecognized flag
and additionally upstream vala added its own meassure to reduce these
new pointer errors to warnings in its generated sources.

23ec71b1a5

Which is part of the current release
branch (https://gitlab.gnome.org/GNOME/vala/-/commits/0.56?ref_type=heads)
and released on 0.56.15

```
Vala 0.56.15
============
 * Various improvements and bug fixes:
  - codegen:
...
    + Emit diagnostic pragmas for GCC 14, Clang 16 compatibility [#1408]
```

https://gitlab.gnome.org/GNOME/vala/-/merge_requests/369

pkgs/development/compilers/vala/setup-hook.sh

@@ -7,15 +7,6 @@ make_vala_find_vapi_files() {
 
 addEnvHooks "$targetOffset" make_vala_find_vapi_files
 
-disable_incompabile_pointer_conversion_warning() {
-    # Work around incompatible function pointer conversion errors with clang 16
-    # by setting ``-Wno-incompatible-function-pointer-types` in an env hook.
-    # See https://gitlab.gnome.org/GNOME/vala/-/issues/1413.
-    NIX_CFLAGS_COMPILE+=" -Wno-incompatible-function-pointer-types"
-}
-
-addEnvHooks "$hostOffset" disable_incompabile_pointer_conversion_warning
-
 _multioutMoveVapiDirs() {
   moveToOutput share/vala/vapi "${!outputDev}"
   moveToOutput share/vala-@apiVersion@/vapi "${!outputDev}"