[mlir][llvm] Clean out f8 types from compatible types list

These were added in faf697e4 so things
can flow through non-opaque LLVM ptrs. Those ptrs are gone so there is
no reason for this to be around anymore. LLVM doesn't support f8 types,
they get converted to i8 when lowering to LLVM dialect.

Removing the f8 types makes LLVM::isCompatibleType and
LLVM::isCompatibleFloatingPointType consistent again.