Remove explicit BPF_CORE_READ

[work-robot]

With a new enough llvm-objdump, we can see the implicitly-generated CO-RE relocations, e.g.:

      74:       if r0 == 0x0 goto +0x28 <LBB0_4>
      75:       r1 = *(u64 *)(r0 + 0x80)
                0000000000000258:  CO-RE <byte_off> [311] struct pt_regs::ip (0:16)
      76:       *(u64 *)(r7 + 0x38) = r1
      77:       r1 = *(u64 *)(r0 + 0x98)
                0000000000000268:  CO-RE <byte_off> [311] struct pt_regs::sp (0:19)
      78:       *(u64 *)(r7 + 0x40) = r1
      79:       r1 = *(u64 *)(r0 + 0x50)
                0000000000000278:  CO-RE <byte_off> [311] struct pt_regs::ax (0:10)
      80:       *(u64 *)(r7 + 0x48) = r1
      81:       r1 = *(u64 *)(r0 + 0x28)
                0000000000000288:  CO-RE <byte_off> [311] struct pt_regs::bx (0:5)
      82:       *(u64 *)(r7 + 0x50) = r1
      83:       r1 = *(u64 *)(r0 + 0x58)
                0000000000000298:  CO-RE <byte_off> [311] struct pt_regs::cx (0:11)
      84:       *(u64 *)(r7 + 0x58) = r1
      85:       r1 = *(u64 *)(r0 + 0x60)
                00000000000002a8:  CO-RE <byte_off> [311] struct pt_regs::dx (0:12)
      86:       *(u64 *)(r7 + 0x60) = r1
      87:       r1 = *(u64 *)(r0 + 0x68)
                00000000000002b8:  CO-RE <byte_off> [311] struct pt_regs::si (0:13)
      88:       *(u64 *)(r7 + 0x68) = r1
      89:       r1 = *(u64 *)(r0 + 0x70)
                00000000000002c8:  CO-RE <byte_off> [311] struct pt_regs::di (0:14)
      90:       *(u64 *)(r7 + 0x70) = r1
      91:       r1 = *(u64 *)(r0 + 0x20)

(struct pt_regs comes from vmlinux.h which has all structs with an implicit preserve_index_access)

[DispatchCode]

Sounds good thanks.