[llvm-c] Remove C API functions that are incompatible with opaque pointers

These deprecated functions are incompatible with opaque pointers,
and have replacements that accept an explicit type. Drop them now
as a final warning to consumers of the C API to migrate their code
(while LLVMGetElementType still exists as a temporary workaround).

Differential Revision: https://reviews.llvm.org/D135271