[mlir][NFC] Remove illegal TanhOp in LLVMConversionTarget

[tpopp]

Summary

No tests fail and this seems to be technical debt from when the math
dialect was created. This should not be there as it prevents users from
configuring their converion target freely and results in unexpected
behavior on seemingly unrelated ops.

[ftynse]

Do this let us also remove the build/link dependency on the math dialect from this conversion?

[tpopp]

In D103388#2788993, @ftynse wrote:

Do this let us also remove the build/link dependency on the math dialect from this conversion?

I just tried, but StandardToLLVM still has many Math lowerings in it

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

void AllocLikeOpLLVMLowering::rewrite(
   // Return the final value of the descriptor.
   rewriter.replaceOp(op, {memRefDescriptor});
 }
 
 mlir::LLVMConversionTarget::LLVMConversionTarget(MLIRContext &ctx)
     : ConversionTarget(ctx) {
   this->addLegalDialect<LLVM::LLVMDialect>();
   this->addIllegalOp<LLVM::DialectCastOp>();
-  this->addIllegalOp<math::TanhOp>();
 }
 
 std::unique_ptr<OperationPass<ModuleOp>> mlir::createLowerToLLVMPass() {
   return std::make_unique<LLVMLoweringPass>();
 }
 
 std::unique_ptr<OperationPass<ModuleOp>>
 mlir::createLowerToLLVMPass(const LowerToLLVMOptions &options) {