RFR: 8313435: Clean up unused default methods code

[Coleen Phillimore]

Default methods processing code has unused code (that gets -Wconversion warnings) from when it was used to create bridge (called overpass) method for an early implementation of generic reification in Hotspot.

This change removes unused bytecodeAssembler code and adds a check for methods and constant pool overflow.
Tested with tier1 and runtime/lambda-features tests.

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Commit messages:
 - 8313435: Clean up unused default methods code

Changes: https://git.openjdk.org/jdk/pull/15095/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15095&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313435
  Stats: 174 lines in 3 files changed: 18 ins; 136 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/15095.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15095/head:pull/15095

PR: https://git.openjdk.org/jdk/pull/15095