[code-reflection] Integrated: Remove need for manual initial creation of backends/cmake-debug-build

[Gary Frost]

Found out how to get maven to create the baclends/cmake-debug-build dir. 

So now we don't need the extra step on initial build. 

Fixes up README.md accordingly

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Commit messages:
 - Remove need for manual initial creation of backends/cmake-debug-build

Changes: https://git.openjdk.org/babylon/pull/131/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=131&range=00
  Stats: 18 lines in 2 files changed: 1 ins; 10 del; 7 mod
  Patch: https://git.openjdk.org/babylon/pull/131.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/131/head:pull/131

PR: https://git.openjdk.org/babylon/pull/131