RFR: 8267317: Remove DeferredTypeCompleter [v2]

[Maurizio Cimadamore]

> This pathc removes DeferredTypeCompleter, which seems like a dubious abstraction. In its place there is now a `complete` method on `DeferredType` which is overridden by `ArgumentType`. The only problematic usage is from `StructuralStuckChecker`, but after inspecting the code I realized that the only side effect this code expected from calling `DeferredType::check` was to set the deferred type mode to `SPECULATIVE`, which we can easily do inline.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Remove redundant assigment of DeferredType::mode

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4091/files
  - new: https://git.openjdk.java.net/jdk/pull/4091/files/c2d4e38c..b0f9ac59

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4091&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4091&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4091.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4091/head:pull/4091

PR: https://git.openjdk.java.net/jdk/pull/4091