Remove unusual use of core_addr_eq and core_addr_hash

gdbtypes.h uses core_addr_eq and core_addr_hash in a weird way: taking
the address of a member and then passing this (as a void*) to these
functions.

It seems better to simply inline the ordinary code here.  CORE_ADDR is
a scalar so it can be directly compared, and the identity hash
function seems safe to assume as well.

After this, core_addr_eq and core_addr_hash are unused, so this patch
removes them.