Remove full redraw user option

[emil-e]

Summary

• remove the ghostel-full-redraw defcustom and documented option
• replace line-mode's buffer-local override with an explicit full-redraw request while line mode is active
• drop tests and benchmark/hypothesis bindings that referenced the option

Verification

• git diff --check
• make byte-compile .build/tests/elisp-ghostel-line-mode-test.ok

Note: a direct byte-compile of bench/ghostel-bench.el and test/hypothesis/ghostel-hypothesis-driver.el still hits pre-existing warnings/errors unrelated to this change.