Replace `--dex-api` with `signature-to-dex` instead

Bug: 307737035
Test: m combined-removed-dex
      # Check before and after to make sure there are no differences
Change-Id: I51e774e6a340d310b96c0291b04f66b4737aebda

api/gen_combined_removed_dex.sh

@@ -6,6 +6,6 @@ shift 2
 
 # Convert each removed.txt to the "dex format" equivalent, and print all output.
 for f in "$@"; do
-    "$metalava_path" "$f" --dex-api "${tmp_dir}/tmp"
+    "$metalava_path" signature-to-dex "$f" "${tmp_dir}/tmp"
     cat "${tmp_dir}/tmp"
 done