AArch64: Delete unused LDATA pseudo-instruction

In other NCG backends, e.g. float/double constants are written to the data section. In AArch64 they are encoded and loaded as immediates. Thus, LDATA is actually unused (i.e. nothing produces them.) This MR removes the dead code.

I had a quick chat with @angerman about the immediate loading way: We assume that it is more cache friendly than loading a constant from an address and should thus stay as it is. If we ever change our minds, we may re-introduce LDATA.