refactor: remove dead shell.c, integrate commands into kconsole

refactor: remove dead shell.c, integrate commands into kconsole

shell.c became dead code after kconsole_enter() replaced shell_init()
as the fallback when init_start() fails. Nobody called shell_init().

- Integrate useful shell commands into kconsole: ls, cat, clear,
  mem, sleep, ring3 (via arch_platform_usermode_test_start)
- Remove src/kernel/shell.c and include/shell.h
- Remove shell.h include from main.c
- kconsole is now the single kernel-mode interactive console

Passes: make, cppcheck, QEMU smoke test.