core/circular_buffer: refactor loop in circular_buffer::erase()
core/circular_buffer: refactor loop in circular_buffer::erase()

* replace `while()` with `for()` for better readability
* use prefix increment operator of iterator for better performance

Signed-off-by: Kefu Chai <cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>

scylladb/seastar,scylladb/seastar,scylladb/seastar