Remove AggressiveOptimization flag from System.Text (#25356)

Remove AggressiveOptimization flag from System.Text (#25356)

Based on conversation with Levi, I don't think this flag is needed.

The flag currently controls two things:
* Disqualifies the method from precompilation
* Disables Tier-0 JIT for the method

Removing the attribute allows us to precompile some of the methods (not all, because of `Vector<T>`). It measurably helps improve startup time because many of the attributed methods are complex and take a while to compile.

Once tier-1 JIT kicks in, the generated code should be identical to what we were getting before.