[code-reflection] RFR: Remove minor unnecessary changes from main javac codebase

[Maurizio Cimadamore]

This PR undoes some unnecessary changes to the javac code:
* `LambdaToMethod` still mentions the `QUOTABLE` flag, which is no longer in use
* `Symtab` has an helper method to load classes from a string name, but that's not really required
* `TreeInfo` defines a new helper function to detect `super` in an expression. Further analysis revealed this was unnecessary (as we can piggy back on the method reference kind instead)

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/711/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=711&range=00
  Stats: 45 lines in 5 files changed: 6 ins; 32 del; 7 mod
  Patch: https://git.openjdk.org/babylon/pull/711.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/711/head:pull/711

PR: https://git.openjdk.org/babylon/pull/711