forgejo: remove no longer needed preBuild phase (noop)

This slipped through the 1.21.11-1 -> 7.0.0 bump in
6c359d6488.

Upstream made this a noop with a deprecation warning instead of a
hard fail in https://codeberg.org/forgejo/forgejo/pulls/2864.

Note: In that PR (it's commit) they link to nixpkgs as downstream
consumer of it (`build/merge-forgejo-locales.go`).

~~~
# Before
Running phase: buildPhase
NOT NEEDED: THIS IS A NOOP AS OF Forgejo 7.0 BUT KEPT FOR BACKWARD COMPATIBILITY
Building subPackage ./.

# After
Running phase: buildPhase
Building subPackage ./.
~~~

Upstream commit: 6647e4d53f

pkgs/by-name/fo/forgejo/package.nix

@@ -80,10 +80,6 @@ buildGoModule rec {
     export ldflags+=" -X main.ForgejoVersion=$(GITEA_VERSION=${version} make show-version-api)"
   '';
 
-  preBuild = ''
-    go run build/merge-forgejo-locales.go
-  '';
-
   postInstall = ''
     mkdir $data
     cp -R ./{templates,options} ${frontend}/public $data
@@ -93,10 +89,9 @@ buildGoModule rec {
       --prefix PATH : ${lib.makeBinPath [ bash git gzip openssh ]}
   '';
 
-  # $data is not available in goModules.drv and preBuild isn't needed
+  # $data is not available in goModules.drv
   overrideModAttrs = (_: {
     postPatch = null;
-    preBuild = null;
   });
 
   passthru = {