Remove document listener from document on handleDispose()

[iloveeclipse]

UpdateDocumentListener is added to document but not always removed in ProjectionViewer, regression introduced by commit
1848058

This fixes only one possible leak, where a single document was used in ProjectionViewer, to unblock JDT UI tests.

Fixes eclipse-jdt/eclipse.jdt.ui#2532

[iloveeclipse]

Github build fails with Error: Failed to execute goal org.eclipse.tycho:tycho-surefire-plugin:5.0.0:test (default-test) on project org.eclipse.core.filebuffers.tests: No tests found.
Fully unrelated...

[iloveeclipse]

Also interesting:

Error:  Failed to execute goal org.eclipse.tycho:tycho-surefire-plugin:5.0.0:test (default-test) on project org.eclipse.e4.core.commands.tests: Could not determine test framework provider. Available providers: [junit5vintageinternal, junit5, junit4, testng, junit5vintage] -> [Help 2]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.eclipse.tycho:tycho-surefire-plugin:5.0.0:test (default-test) on project org.eclipse.e4.core.commands.tests: Could not determine test framework provider. Available providers: [junit5vintageinternal, junit5, junit4, testng, junit5vintage]`

Fully unrelated...

@laeubi : is something changed in tycho?
Could be related to both JUnit 5 & JUnit6 available on the IBuild repo?
See https://download.eclipse.org/eclipse/updates/4.38-I-builds/I20251007-1800/plugins/ and
eclipse-platform/eclipse.platform.releng.aggregator#3399

[github-actions]

Test Results

  603 files  ±0    603 suites  ±0   1m 55s ⏱️ +9s
1 328 tests ±0  1 325 ✅ ±0   3 💤 ±0  0 ❌ ±0 
4 032 runs  ±0  4 021 ✅ ±0  11 💤 ±0  0 ❌ ±0 

Results for commit 2c0904e. ± Comparison against base commit d77df82.

[iloveeclipse]

All errors are about not found tests and all are unrelated, change works locally, so let's merge.