Remove apex_pubkey from the bundle module

It's not yet supported by bundletool and blocks other testing (see
attached bug).

This is a partial revert of
https://android-review.googlesource.com/c/platform/build/soong/+/1183459/

Test: out/dist/mainline_modules_arm64/com.android.tzdata-base.zip
Test: checked content doesn't have apex_pubkey
Bug: 146460014
Change-Id: I268caa9f9736ba1913145a1ce1065f6f5179a6db

apex/builder.go

@@ -138,7 +138,6 @@ var (
 		Command: `${zip2zip} -i $in -o $out ` +
 			`apex_payload.img:apex/${abi}.img ` +
 			`apex_manifest.json:root/apex_manifest.json ` +
-			`apex_pubkey:root/apex_pubkey ` +
 			`apex_manifest.pb:root/apex_manifest.pb ` +
 			`AndroidManifest.xml:manifest/AndroidManifest.xml ` +
 			`assets/NOTICE.html.gz:assets/NOTICE.html.gz`,