apparmor: Remove SECURITY_APPARMOR_BOOTPARAM_VALUE

In preparation for removing CONFIG_DEFAULT_SECURITY, this removes the
soon-to-be redundant SECURITY_APPARMOR_BOOTPARAM_VALUE. Since explicit
ordering via CONFIG_LSM or "lsm=" will define whether an LSM is enabled or
not, this CONFIG will become effectively ignored, so remove it. However,
in order to stay backward-compatible with "security=apparmor", the enable
variable defaults to true.

Signed-off-by: Kees Cook <keescook@chromium.org>