fix: prevent terminal panel from overwriting terminalEditorActive context key (#304802)

The terminalEditorActive context key was being incorrectly set to false
when a terminal in the panel received focus, even though the active
editor was still a terminal editor. This happened because
TerminalService listened to onDidChangeActiveInstance and set the key
based on the active terminal instance's location rather than whether the
active editor is a terminal editor.

Remove the duplicate terminalEditorActive management from
TerminalService since TerminalEditorService already correctly manages
this context key via onDidActiveEditorChange, which checks whether the
active editor is a TerminalEditorInput.

Closes #182979