lib: delete dead code in SourceMap

This seems to be a leftover from the chromium project. Nothing uses
`#reverseMappingsBySourceURL`, so constructing it isn't necessary.

PR-URL: nodejs#31512
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/source_map/source_map.js

@@ -121,7 +121,6 @@ class StringCharIterator {
  */
 class SourceMap {
   #payload;
-  #reverseMappingsBySourceURL = [];
   #mappings = [];
   #sources = {};
   #sourceContentByURL = {};
@@ -262,19 +261,6 @@ class SourceMap {
       this.#mappings.push([lineNumber, columnNumber, sourceURL,
                            sourceLineNumber, sourceColumnNumber]);
     }
-
-    for (let i = 0; i < this.#mappings.length; ++i) {
-      const mapping = this.#mappings[i];
-      const url = mapping[2];
-      if (!url)
-        continue;
-      if (!this.#reverseMappingsBySourceURL[url])
-        this.#reverseMappingsBySourceURL[url] = [];
-      const reverseMappings = this.#reverseMappingsBySourceURL[url];
-      const sourceLine = mapping[3];
-      if (!reverseMappings[sourceLine])
-        reverseMappings[sourceLine] = [mapping[0], mapping[1]];
-    }
   };
 }