Make the DefineSymbol not auto update after reload scripts.

[DarthCY-K]

This method consumed a lot of cpu times(more than 10000ms, 120Mb GC) after my scripts reloaded in a huge VR project based on Unity2019.4.40f1, windows10 with I7-8100 CPU and 16G RAM. It freezes my unity anytime when script reloaded, causing bad experience in developing. I think it's not necessary to do this after any scripts reload, just use it after switch platform, and manual reload by developers.