Removed no longer supported call to in_eager_execution

changes to model_analyser.analyse(...):
- Swapped context.in_eager_execution() to the currently supported context.executing_eagerly().
- Added negation to eager check. In all likelihood, the negation was always supposed to be there since getting default graph in eager mode does not make sense. The current `if` condition is likely a bug. The proposed fix is consistent with other functions in this module, e.g., `profile(...)`, line 339.