Fix crash on text hint being triggered after disabling code browser plugin

From KTextEditor::TextHintInterface::registerTextHintProvider() docs:
"When the provider is about to be destroyed, make sure to call
unregisterTextHintProvider() to avoid a dangling pointer."
(new requirement with KF5)

The old code never unregistered in case the ContextBrowserPlugin was
deleted (like it happens when disabling the plugin in the settings in
a running KDevelop instance).
1 job for !56 with ficrashoncodebrowserplugindisabling in 60 minutes (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
failed #16065
linux kf5-qt5 qt5.12

01:00:00

 
Name Stage Failure
failed
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
qt.qpa.events: Event | XCB_LEAVE_NOTIFY(8) | sequence: 400
qt.qpa.events: Event | XCB_PROPERTY_NOTIFY(28) | sequence: 400
qt.qpa.events: Event | XCB_PROPERTY_NOTIFY(28) | sequence: 400
qt.qpa.events: Event | XCB_PROPERTY_NOTIFY(28) | sequence: 400
qt.qpa.events: Event | XCB_ENTER_NOTIFY(7) | sequence: 400
qt.qpa.events: Event | XCB_PROPERTY_NOTIFY(28) | sequence: 400
qt.qpa.events: Event | XCB_PROPERTY_NOTIFY(28) | sequence: 400
Pulling docker image gitlab/gitlab-runner-helper:x86_64-ac2a293c ...
ERROR: Job failed: execution took longer than 1h0m0s seconds