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: https://github.com/nodejs/node/pull/31512


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