Temporarily disable optimized batched solver instantiations

[pratikvn]

This PR temporarily disables the instantiations of the different batched solver kernels to reduce compilation time.

[MarcelKoch]

nit: IMO it could just be deleted directly, we still have the code in the git log, so it won't be lost. But I'm also fine with leaving it as it is, which probably highlights that it's only temporary.

[upsj]

I feel the same way, but since this is temporary, it shouldn't matter much

[yhmtsai]

It currently forces the behavior.
Because you add the config.hpp, I thought you would like to use config.hpp (configured by CMake) to control the behavior.
Because it is used by some projects, do they use these optimized instantiation?

[pratikvn]

Yes, currently we decided to temporarily remove the optimized instantiations. A more permanent solution would be something of the form in #1629

We decided not to go with the config options as that is maybe more annoying for users.

Currently a few projects use the optimized versions, but I think they rely on the previous release, so it should be okay. We will move to the split version soon anyway.

[yhmtsai]

LGTM. The config header is unnecessary now.
How do we ensure the optimized version is still enabled in the next release? maybe make some note in changelog or create an issue until the splitting compilation pr is merged.

[pratikvn]

Created an issue #1661, with the milestone 1.9.0, to track it.