[flang] Removed dynamic loading feature for intrinsic folding

After more reflexion, dynamic loading brings to much uncertainty
regarding which library is actually being use for folding.
It is removed to avoid pushing people to use it.
A static linking to libpgmath will be provided in a later commit.

Original-commit: flang-compiler/f18@2161627d2896f06e763848443f61ae9624f03db1
Tree-same-pre-rewrite: false