[JITLink][AArch64] Remove unused Pointer64Anon edge kind.

Pointer64Anon was lifted out of the MachO backend and into aarch64.h when that
header was created, but Pointer64Anon is really a MachO-specific "normalized"
relocation value, rather than a generic Edge::Kind. Any uses can be safely
replaced with Pointer64.

(Side note: the role of MachOPointer64Anon is to aid MachO relocation parsing:
For MachOPointer64, the target symbol is specified by the r_symbolnum field in
the relocation. For MachOPointer64Anon the address of the anonymous target is
read from the fixup location.)