GenerateMenuMT: technical changes to ease profiling

• Remove unnecessary memoize decorators in GenerateMenuMT (none of these functions are called more than once).
• Remove all uses of eval to make the code more friendly to the python profiler (see below).
• Remove the do[SIG]Chains flags as they were not being used. If this feature is needed they should instead be implemented as a dictionary.
• Replace the use of __import__ with importlib.

Relates to ATR-23767.

Before	After