Incorrect world destruction fix for FlecsSubsystem.cpp

[VergilUa]

ECSWorld was never destroyed, which lead to Rest API module getting stuck and potentially other side-effects.

WTR: Without rebuilding / restarting editor:

• Start the game;
• Stop the game;
• Start the game;

ER:

• Explorer finds the world correctly;

AR:

• Rest API is unavailable, because the same port is already occupied by undisposed world.

[PreyK]

Good catch! Thanks for the PR!