config: drop kernel config symbols removed before v6.6

config: drop kernel config symbols removed before v6.6

Since we don't support any kernel versions below v6.6, Kconfig options
that were removed upstream before 6.6 are no longer relevant and should be
dropped.

This commit removes the following obsolete kernel config options:
- KERNEL_UBSAN_NULL: Removed in v4.18 (3ca17b1f3628)
- KERNEL_DEBUG_LL_UART_NONE: Removed in v4.0 (6f5194553c84)
- KERNEL_SLABINFO: Removed in v4.15 (5b36577109be)
- KERNEL_RESOURCE_COUNTERS: Removed in v3.18 (5b1efc027c0b)
- KERNEL_MM_OWNER: Removed in v3.16 (f98bafa06a28)
- KERNEL_MEMCG_SWAP: Removed in v6.1 (e55b9f96860f)
- KERNEL_MEMCG_SWAP_ENABLED: Removed in v5.7 (2d1c498072de)
- KERNEL_DEVPTS_MULTIPLE_INSTANCES: Removed in v4.7 (eedf265aa003)
- KERNEL_DEVKMEM: Removed in v5.13 (bbcd53c96071)
- KERNEL_SECURITY_SELINUX_DISABLE: Removed in v6.4 (f22f9aaf6c3d)

Signed-off-by: Harrison Boyd <harrisonboyd400@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19287
Signed-off-by: Robert Marko <robimarko@gmail.com>