Remove at-fastmath from base

[musm]

Ref. comments from issue, which echo my feelings as well:

Getting rid of @fastmath might be good, not because of this issue, but because it's too loosely defined. People use it for all sort of different reasons (which all make code run "fast"). But before doing/evaluating that the replacement should go in first. e.g. #31862 and from the way people would like to use these this is certainly not trivial, and it'll likely always involve authors of the math function to do the right thing.
@yuyichao

There are other issues: @fastmath operates syntactically which can cause unexpected reults (#26828); --math-mode=fast doesn't do syntactic rewriting, instead just enables the compiler optimizations, which leads to other bizarre behavior (#30073). ..... Both are giant footguns.
@simonbyrne

It seems like this could only go in 2.0

[StefanKarpinski]

We could make it a no-op pre 2.0 though since code would still work.