Debugger: Remove the RebuildBreakpointModel command flags

This was useful to trigger collective attemptBreakpointSync() in the gdb
engine when gdb did not yet track pending breakpoints by itself.

That's not needed anymore.

Change-Id: I8afd2955070504c6bd6ae6c028b3f006b6eb4d7c
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/gdb/gdbengine.h

@@ -159,8 +159,6 @@ private: ////////// General Interface //////////
     int m_oldestAcceptableToken = -1;
     int m_nonDiscardableCount = 0;
 
-    int m_pendingBreakpointRequests = 0; // Watch updating commands in flight
-
     typedef void (GdbEngine::*CommandsDoneCallback)();
     // This function is called after all previous responses have been received.
     CommandsDoneCallback m_commandsDoneCallback = nullptr;