RFR: Use method offset nmethods hash

[Ashutosh Mehra]

Currently AOTCodeEntry for nmethod uses a hash obtained from the method holder's name, method name and signature.   This is not very reliable.  With https://github.com/openjdk/leyden/pull/90, AOTCodeEntry is always linked to its corresponding Method, so we should be able to use the Method pointer as the hash for AOTCodeEntries for nmethod. However, the problem with pointers are not stable across runs. So instead of the pointer, we can use the offset of the Method in AOTCache. This should work because the Method is guaranteed to be in AOTCache.
This PR updates the code to use Method offset as the hash for AOTCodeEntries for nmethods.

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Commit messages:
 - Remove unused code
 - Use method offset as the hash for storing nmethods

Changes: https://git.openjdk.org/leyden/pull/91/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=91&range=00
  Stats: 23 lines in 1 file changed: 1 ins; 16 del; 6 mod
  Patch: https://git.openjdk.org/leyden/pull/91.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/91/head:pull/91

PR: https://git.openjdk.org/leyden/pull/91