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/debuggerprotocol.h

@@ -84,8 +84,6 @@ public:
         ExitRequest = 32,
         // Auto-set inferior shutdown related states.
         LosesChild = 64,
-        // Trigger breakpoint model rebuild when no such commands are pending anymore.
-        RebuildBreakpointModel = 128,
         // This is a native (non-Python) command that's handled directly by the backend.
         NativeCommand = 256,
         // This is a command that needs to be wrapped into -interpreter-exec console