compiler: remove unused GHC.Linker.Loader.loadExpr

This patch removes the unused `GHC.Linker.Loader.loadExpr` function.
It was moved from `GHC.Runtime.Linker.linkExpr` in `ghc-9.0` to
`GHC.Linker.Loader.loadExpr` in `ghc-9.2`, and remain completely
unused and untested ever since. There's also no third party user of
this function to my best knowledge, so let's remove this. Anyone who
wants to write their own GHC API function to load bytecode can consult
the source code in older release branches.

(cherry picked from commit 536cdf09)