bpf: remove {update,get}_loop_entry functions

bpf: remove {update,get}_loop_entry functions

The previous patch switched read and precision tracking for
iterator-based loops from state-graph-based loop tracking to
control-flow-graph-based loop tracking.

This patch removes the now-unused `update_loop_entry()` and
`get_loop_entry()` functions, which were part of the state-graph-based
logic.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250611200836.4135542-9-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>